script: Add support for the accesskey attribute (#43031)

This change adds support for the `accesskey` HTML attribute the
corresponding behavior defined by the HTML standard. In addition to a
new map holding a mapping between access key characters and a rooted
reference to elements that use them, it adds a struct to represent the
"Command" concept from the specification [^1], which seems to be used
for `accesskey`. Not all command facets are implemented as some seem to
be unused in the specification.

[^1]:
https://html.spec.whatwg.org/multipage/interactive-elements.html#commands

Testing: This causes most access key tests to start passing. One test
now
only partially completes, because of a bug in the way that we fire
synthetic
mouse events.
Fixes: This is part of #25001.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/html/htmloptionelement.rs

@@ -155,7 +155,7 @@ impl HTMLOptionElement {
     }
 
     /// <https://html.spec.whatwg.org/multipage/#option-element-nearest-ancestor-select>
-    fn nearest_ancestor_select(&self) -> Option<DomRoot<HTMLSelectElement>> {
+    pub(crate) fn nearest_ancestor_select(&self) -> Option<DomRoot<HTMLSelectElement>> {
         // Step 1. Let ancestorOptgroup be null.
         // NOTE: We only care whether the value is non-null, so a boolean is enough
         let mut did_see_ancestor_optgroup = false;