LibWeb: Handle CSS "ch" length unit (mostly)

This isn't 100% spec complaint, as it should use glyph_height()
depending on what the value of the writing-mode is, but we haven't
implemented it yet, so I think it'll be good enough for now.

This can be tested in https://wpt.live/css/css-values/ch-unit-008.html
Other css-unit tests fail as:
- 001 shows an issue related to a renderer (looks to me like you can't
  pass a width and height property to a span -- adding `display: block`
  to it passes the test),
- 002-004 and 009-012 use mentioned writing-mode,
- 016-017 loads custom fonts, which we also don't support (yet).

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -1466,6 +1466,8 @@ Optional<Length> Parser::parse_length(ParsingContext const& context, StyleCompon
             type = Length::Type::Em;
         } else if (unit_string.equals_ignoring_case("ex")) {
             type = Length::Type::Ex;
+        } else if (unit_string.equals_ignoring_case("ch")) {
+            type = Length::Type::Ch;
         } else if (unit_string.equals_ignoring_case("vw")) {
             type = Length::Type::Vw;
         } else if (unit_string.equals_ignoring_case("vh")) {