Add validation for internal binding functions

[dario-piotrowicz]

When a function is added to one of the internal bindings its types should be added to the internalBindings typings (so that the type can be picked up by IDEs and be reflected in the lib javascript files).

Currently (as far as I know) there is no type checking being applied to the js files, so it's pretty easy to forget to add the appropriate typings.

Ideally a process should be put in place to validate that all types defined in internal bindings are present in the typings directory (matching the idea described in the onboarding documentation that tooling should identify PR issues when possible)

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