Move the remaining CSSStyleDeclaration property-name APIs to
Utf16FlyString. This lets CSSOM binding and generated accessor code
pass JS property names without first constructing FlyString values.
Keep internal custom-property and descriptor storage unchanged for now.
Those remaining FlyString conversions are at storage boundaries that
will be migrated in follow-up commits.
Change get_property_value() to take a Utf16FlyString so generated CSS
property accessors can pass their JS property names through without
constructing FlyString instances first.
Keep the existing internal descriptor and custom-property storage shape
for now, and convert at those boundaries while the remaining CSS
property APIs are migrated separately.
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.
