style: Allow to derive Parse/ToCss/SpecifiedValueInfo on bitflags

We keep getting this pattern of properties that have a set of joint and
disjoint flags, and copy-pasting or writing the same parsing and
serialization code in slightly different ways.

container-type is one such type, and I think we should have a single way
of dealing with this, thus implement deriving for various traits for
bitflags, with an attribute that says which flags are single vs mixed.

See docs and properties I ported. The remaining ones I left TODOs with,
they are a bit trickier but can be ported with some care.

Differential Revision: https://phabricator.services.mozilla.com/D142418

components/derive_common/cg.rs

@@ -373,6 +373,11 @@ pub fn to_css_identifier(mut camel_case: &str) -> String {
     result
 }
 
+/// Transforms foo-bar to FOO_BAR.
+pub fn to_scream_case(css_case: &str) -> String {
+    css_case.to_uppercase().replace('-', "_")
+}
+
 /// Given "FooBar", returns "Foo" and sets `camel_case` to "Bar".
 fn split_camel_segment<'input>(camel_case: &mut &'input str) -> Option<&'input str> {
     let index = match camel_case.chars().next() {