Make "theia" and "vscode" contributed API's

[tsmaeder]

Currently, the theia and vscode API namespaces are special in that they are not contributed via the ExtPluginApiProvider contribution point, but hard-wired in.
I think it would make the code easier to understand if we contributed those API namespaces just like any other API extensions.
Also, it would make sure third-party API have everything needed (currently, initialization protocol is different, so there is some stuff you might not be able to do in a contributed API).
Thirdly, we could separate plugin-ext and plugin-ext theia, thus further separating concerns and aiding understanding.

[tsmaeder]

As a further plus, plugins could use both the vscode and the theia namespaces at the same time, which is not possible at this time. There has been discussions about the theia namespace just containing API that is not the same as vscode API.

[Pines-Cheng]

The PR seems like still not merged.