LibWeb: Add “valid floating-point number” for HTMLInputElement.value

This change adds checking for the following spec requirements:

- https://html.spec.whatwg.org/#number-state-(type=number):value-sanitization-algorithm
- https://html.spec.whatwg.org/#range-state-(type=range):value-sanitization-algorithm

That is, it adds checking that HTMLInputElement.value is what the spec
defines as a “valid floating-point number” when the “type” attribute for
the HTMLInputElement is either “number” or “range”.

This change causes Ladybird to pass all the failing tests at
https://wpt.fyi/results/html/semantics/forms/the-input-element/number.html?run_id=5080423051034624
and to match the relevant behavior in Webkit, Blink, and Gecko.

Otherwise, without this change, Ladybird fails those tests, and the
relevant Ladybird behavior isn’t interoperable with other engines.

Userland/Libraries/LibWeb/HTML/Numbers.cpp

@@ -92,6 +92,39 @@ Optional<double> parse_floating_point_number(StringView string)
     return maybe_double.value();
 }
 
+// https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#valid-floating-point-number
+bool is_valid_floating_point_number(StringView string)
+{
+    GenericLexer lexer { string };
+    // 1. Optionally, a U+002D HYPHEN-MINUS character (-).
+    lexer.consume_specific('-');
+    // 2. One or both of the following, in the given order:
+    // 2.1. A series of one or more ASCII digits.
+    bool has_leading_digits = !lexer.consume_while(is_ascii_digit).is_empty();
+    // 2.2. Both of the following, in the given order:
+    // 2.2.1. A single U+002E FULL STOP character (.).
+    if (lexer.consume_specific('.')) {
+        // 2.2.2. A series of one or more ASCII digits.
+        if (lexer.consume_while(is_ascii_digit).is_empty())
+            return false;
+    } else if (!has_leading_digits) {
+        // Doesn’t begin with digits, doesn’t begin with a full stop followed by digits.
+        return false;
+    }
+    // 3. Optionally:
+    // 3.1. Either a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL
+    //      LETTER E character (E).
+    if (lexer.consume_specific('e') || lexer.consume_specific('E')) {
+        // 3.2. Optionally, a U+002D HYPHEN-MINUS character (-) or U+002B PLUS SIGN
+        //      character (+).
+        lexer.consume_specific('-') || lexer.consume_specific('+');
+        // 3.3. A series of one or more ASCII digits.
+        if (lexer.consume_while(is_ascii_digit).is_empty())
+            return false;
+    }
+    return lexer.tell_remaining() == 0;
+}
+
 WebIDL::ExceptionOr<String> convert_non_negative_integer_to_string(JS::Realm& realm, WebIDL::Long value)
 {
     if (value < 0)