AK+Everywhere: Replace custom number parsers with fast_float

Our floating point number parser was based on the fast_float library:
https://github.com/fastfloat/fast_float

However, our implementation only supports 8-bit characters. To support
UTF-16, we will need to be able to convert char16_t-based strings to
numbers as well. This works out-of-the-box with fast_float.

We can also use fast_float for integer parsing.

AK/StringConversions.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2022, David Tuin <davidot@serenityos.org>
+ * Copyright (c) 2025, Tim Flynn <trflynn89@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Concepts.h>
+#include <AK/Optional.h>
+#include <AK/StringUtils.h>
+
+namespace AK {
+
+template<typename T>
+struct ParseFirstNumberResult {
+    T value { 0 };
+    size_t characters_parsed { 0 };
+};
+
+template<Arithmetic T>
+Optional<ParseFirstNumberResult<T>> parse_first_number(StringView, TrimWhitespace = TrimWhitespace::Yes, int base = 10);
+
+template<Arithmetic T>
+Optional<T> parse_number(StringView, TrimWhitespace = TrimWhitespace::Yes, int base = 10);
+
+template<Integral T>
+Optional<T> parse_hexadecimal_number(StringView, TrimWhitespace = TrimWhitespace::Yes);
+
+}