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/style_traits/values.rs

@@ -44,6 +44,29 @@ use std::fmt::{self, Write};
 /// * `#[css(represents_keyword)]` can be used on bool fields in order to
 ///   serialize the field name if the field is true, or nothing otherwise.  It
 ///   also collects those keywords for `SpecifiedValueInfo`.
+/// * `#[css(bitflags(single="", mixed="", validate="")]` can be used to derive
+///   parse / serialize / etc on bitflags. The rules for parsing bitflags are
+///   the following:
+///
+///     * `single` flags can only appear on their own. It's common that bitflags
+///       properties at least have one such value like `none` or `auto`.
+///     * `mixed` properties can appear mixed together, but not along any other
+///       flag that shares a bit with itself. For example, if you have three
+///       bitflags like:
+///
+///         FOO = 1 << 0;
+///         BAR = 1 << 1;
+///         BAZ = 1 << 2;
+///         BAZZ = BAR | BAZ;
+///
+///       Then the following combinations won't be valid:
+///
+///         * foo foo: (every flag shares a bit with itself)
+///         * bar bazz: (bazz shares a bit with bar)
+///
+///       But `bar baz` will be valid, as they don't share bits, and so would
+///       `foo` with any other flag, or `bazz` on its own.
+///
 /// * finally, one can put `#[css(derive_debug)]` on the whole type, to
 ///   implement `Debug` by a single call to `ToCss::to_css`.
 pub trait ToCss {