Make theia.API an additional namespace

[tsmaeder]

Currently, you can have either the theia or the vscode namespace and the are the same, save for a couple of renamings ('plugin'=>'extension'). I think it would make things simpler if we only had the vscode namespace and the theia namespace would only contain functionality that is separate of what is already in vscode namespace.

This would entail to either make all API namespaces contributed (see #8142) or at least make the one of them the default and the other a contributed one. The problem here is that currently, we're using the packaging format to determine which default API to pass onto a particular plugin.

This change would allow us to get rid of our own package format and to just rely on the vs code packaging format and tooling. We would also end up with a better separation of concerns in the plugin implementation package: we would end up with four packages:

• plugin
  Contains just the mechanism for having plugin API namespaces
• plugin-ext-vscode
  Provides the contributed vscode API namespace
• theia
  Dev-time package containing theia.d.ts
• plugin-ext-theia
  Dev-time package containing the contributed theia API namespace.