Implement ::part selector support

[simonwuelker]

Companion PR for servo/servo#37307

[simonwuelker]

This can be merged now that servo/servo#37307 is approved.

[Loirooriol]

Might have been good to raise the version of web_atoms to 0.1.3 in Cargo.toml, I was confused about not being able to compile Stylo.

[simonwuelker]

Might have been good to raise the version of web_atoms to 0.1.3 in Cargo.toml, I was confused about not being able to compile Stylo.

Makes sense, we should just pin the exact version instead of only specifing the minor release in

stylo/style/Cargo.toml

Line 76 in 3a3663a

web_atoms = { version = "0.1", optional = true }