Previously, the LibWeb bindings generator would output multiple per
interface files like Prototype/Constructor/Namespace/GlobalMixin
depending on the contents of that IDL file.
This complicates the build system as it means that it does not know
what files will be generated without knowledge of the contents of that
IDL file.
Instead, for each IDL file only generate a single Bindings/<IDLFile>.h
and Bindings/<IDLFile>.cpp.
We now clear the computed font cache whenever a `@font-feature-values`
rule is added, removed, or has one of it's descriptors modified.
This isn't observable yet since we don't actually respect
`@font-feature-values` rules, but that will come in a later commit
