feat: Implement @internal as synonym of @hidden

[BurtHarris]

Fixes #364 The @internal doc flag recognised by tsc should be an alias for @hidden
Fixes #198 Support for @ignore?

[aciccarello]

Thanks for the PR! I do think we need to think through if @internal should always be hidden. I know that @internal has it's own meaning outside of TypeDoc. Is there anything in the semantics of that tag that would conflict with documentation?

[BurtHarris]

@aciccarello Having thought about it some myself, this could get as complicated as you want to make it, especially since TypeScript's support for @internal is still experimental. I don't think there's any shame in treating its support in typedoc as experimental.

For my project the documentation spew from typedoc is already enormous (primarily because it was ported from an existing Java class library with extensive doc comments. and I'm looking for any way I can to narrow what's generated to a limited "public" API in the semver sense.

Typedoc already seems to go digging much deeper than is justified by the façade module I pass it. I often end up with documentation related to test case classes and benchmarks, which make no sense to include in the public API.

[Gerrit0]

Closing as this was implemented in #1114, which has now been merged + published. Somehow I missed this PR when making that..