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/Length.h

@@ -27,6 +27,7 @@ public:
         Pc,
         Ex,
         Em,
+        Ch,
         Rem,
         Vh,
         Vw,
@@ -93,6 +94,7 @@ public:
     {
         return m_type == Type::Ex
             || m_type == Type::Em
+            || m_type == Type::Ch
             || m_type == Type::Rem
             || m_type == Type::Vh
             || m_type == Type::Vw