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.

Userland/Libraries/LibWeb/CSS/MediaQuery.cpp

@@ -25,7 +25,7 @@ NonnullRefPtr<MediaQuery> MediaQuery::create_not_all()
 String MediaFeatureValue::to_string() const
 {
     return m_value.visit(
-        [](ValueID const& ident) { return MUST(String::from_utf8(string_from_value_id(ident))); },
+        [](Keyword const& ident) { return MUST(String::from_utf8(string_from_keyword(ident))); },
         [](Length const& length) { return length.to_string(); },
         [](Ratio const& ratio) { return ratio.to_string(); },
         [](Resolution const& resolution) { return resolution.to_string(); },
@@ -35,7 +35,7 @@ String MediaFeatureValue::to_string() const
 bool MediaFeatureValue::is_same_type(MediaFeatureValue const& other) const
 {
     return m_value.visit(
-        [&](ValueID const&) { return other.is_ident(); },
+        [&](Keyword const&) { return other.is_ident(); },
         [&](Length const&) { return other.is_length(); },
         [&](Ratio const&) { return other.is_ratio(); },
         [&](Resolution const&) { return other.is_resolution(); },
@@ -100,10 +100,10 @@ bool MediaFeature::evaluate(HTML::Window const& window) const
         if (queried_value.is_ident()) {
             // NOTE: It is not technically correct to always treat `no-preference` as false, but every
             //       media-feature that accepts it as a value treats it as false, so good enough. :^)
-            //       If other features gain this property for other identifiers in the future, we can
+            //       If other features gain this property for other keywords in the future, we can
             //       add more robust handling for them then.
-            return queried_value.ident() != ValueID::None
-                && queried_value.ident() != ValueID::NoPreference;
+            return queried_value.ident() != Keyword::None
+                && queried_value.ident() != Keyword::NoPreference;
         }
         return false;