Allow the WebIDL parser to accept files without a top-level interface,
and have the exposed-interface generator skip those modules.
With that in place, register support IDLs through
libweb_js_bindings() as well, including generated CSS IDLs from the
build directory, and remove the separate support-idl plumbing.
This is enabled now that the IDL generator rules have been simplified
so that every IDL file produces a corresponding header and cpp file.
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