LibWeb: Rename "identifier" and "ValueID" to "Keyword" where correct

For a long time, we've used two terms, inconsistently:
- "Identifier" is a spec term, but refers to a sequence of alphanumeric
  characters, which may or may not be a keyword. (Keywords are a
  subset of all identifiers.)
- "ValueID" is entirely non-spec, and is directly called a "keyword" in
  the CSS specs.

So to avoid confusion as much as possible, let's align with the spec
terminology. I've attempted to change variable names as well, but
obviously we use Keywords in a lot of places in LibWeb and so I may
have missed some.

One exception is that I've not renamed "valid-identifiers" in
Properties.json... I'd like to combine that and the "valid-types" array
together eventually, so there's no benefit to doing an extra rename
now.

(cherry picked from commit 6a74b0164423d63904cf5a5b594772b595f57600;
very minorly amended to fix conflict in GenerateCSSKeyword.cpp caused
by #22870, and in libweb_generators.cmake due to us not having
https://github.com/LadybirdBrowser/ladybird/pull/741)

Userland/Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.cpp

@@ -100,7 +100,7 @@ static NonnullRefPtr<CSSStyleValue const> style_value_for_background_property(La
 static NonnullRefPtr<CSSStyleValue const> style_value_for_length_percentage(LengthPercentage const& length_percentage)
 {
     if (length_percentage.is_auto())
-        return CSSKeywordValue::create(ValueID::Auto);
+        return CSSKeywordValue::create(Keyword::Auto);
     if (length_percentage.is_percentage())
         return PercentageStyleValue::create(length_percentage.percentage());
     if (length_percentage.is_length())
@@ -111,22 +111,22 @@ static NonnullRefPtr<CSSStyleValue const> style_value_for_length_percentage(Leng
 static NonnullRefPtr<CSSStyleValue const> style_value_for_size(Size const& size)
 {
     if (size.is_none())
-        return CSSKeywordValue::create(ValueID::None);
+        return CSSKeywordValue::create(Keyword::None);
     if (size.is_percentage())
         return PercentageStyleValue::create(size.percentage());
     if (size.is_length())
         return LengthStyleValue::create(size.length());
     if (size.is_auto())
-        return CSSKeywordValue::create(ValueID::Auto);
+        return CSSKeywordValue::create(Keyword::Auto);
     if (size.is_calculated())
         return size.calculated();
     if (size.is_min_content())
-        return CSSKeywordValue::create(ValueID::MinContent);
+        return CSSKeywordValue::create(Keyword::MinContent);
     if (size.is_max_content())
-        return CSSKeywordValue::create(ValueID::MaxContent);
+        return CSSKeywordValue::create(Keyword::MaxContent);
     // FIXME: Support fit-content(<length>)
     if (size.is_fit_content())
-        return CSSKeywordValue::create(ValueID::FitContent);
+        return CSSKeywordValue::create(Keyword::FitContent);
     TODO();
 }
 
@@ -172,7 +172,7 @@ static RefPtr<CSSStyleValue const> style_value_for_length_box_logical_side(Layou
 static RefPtr<CSSStyleValue const> style_value_for_shadow(Vector<ShadowData> const& shadow_data)
 {
     if (shadow_data.is_empty())
-        return CSSKeywordValue::create(ValueID::None);
+        return CSSKeywordValue::create(Keyword::None);
 
     auto make_shadow_style_value = [](ShadowData const& shadow) {
         return ShadowStyleValue::create(
@@ -263,7 +263,7 @@ RefPtr<CSSStyleValue const> ResolvedCSSStyleDeclaration::style_value_for_propert
         //    The resolved value is normal if the computed value is normal, or the used value otherwise.
     case PropertyID::LineHeight: {
         auto line_height = get_computed_value(property_id);
-        if (line_height->is_keyword() && line_height->to_identifier() == ValueID::Normal)
+        if (line_height->is_keyword() && line_height->to_keyword() == Keyword::Normal)
             return line_height;
         return LengthStyleValue::create(Length::make_px(layout_node.computed_values().line_height()));
     }
@@ -370,7 +370,7 @@ RefPtr<CSSStyleValue const> ResolvedCSSStyleDeclaration::style_value_for_propert
     case PropertyID::Transform: {
         auto transformations = layout_node.computed_values().transformations();
         if (transformations.is_empty())
-            return CSSKeywordValue::create(ValueID::None);
+            return CSSKeywordValue::create(Keyword::None);
 
         // https://drafts.csswg.org/css-transforms-2/#serialization-of-the-computed-value
         // The transform property is a resolved value special case property. [CSSOM]
@@ -516,7 +516,7 @@ RefPtr<CSSStyleValue const> ResolvedCSSStyleDeclaration::style_value_for_propert
     case PropertyID::WebkitTextFillColor:
         return ColorStyleValue::create(layout_node.computed_values().webkit_text_fill_color());
     case PropertyID::Invalid:
-        return CSSKeywordValue::create(ValueID::Invalid);
+        return CSSKeywordValue::create(Keyword::Invalid);
     case PropertyID::Custom:
         dbgln_if(LIBWEB_CSS_DEBUG, "Computed style for custom properties was requested (?)");
         return nullptr;