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.
This is a base type for the various transform functions.
CSSMatrixComponent's to_string() can throw exceptions, so to_string() is
implemented that way. https://github.com/w3c/fxtf-drafts/issues/611
+9 WPT subtests.
