* src/CMakeLists.txt: Also generate a custom Also generate a custom
  webkit2gtk-web-extension-4.0 VAPI that re-includes javascriptcore
  objects, so they can be access from the web extension.

* bindings/metadata/WebKit2WebExtension-4.0-custom.vala,
  bindings/metadata/WebKit2WebExtension-4.0.metadata: Include in-tree
  metadata for web extension VAPI, update to all access to JS obejcts.

* bindings/vapi/javascriptcore-4.0.vapi: Add a bunch of useful additional
  objects and method.