• make TheiaVariableContribution more easily to customize/override

fixes #14817

What it does

• Bind TheiaVariableContribution to self, to make it easier to override

Note: the ticket was about integrating the AI Variable Service with the Theia VariableRegistry (ideally reusing the VariableRegistry directly for AI). However, there are some key differences between the two:

• AI Variable Service is part of Common, whereas VariableRegistry only exists in the Frontend
• AI Variable Service is slightly more customizable, as Variable Resolvers are separate from the Variable Definition, and can be overridden more easily
• VariableRegistry supports more types (including structured arrays/objects); AI Variable Service only supports strings

Moreover, the AI Variable Service already includes a Contribution that delegates to the VariableRegistry in the browser, so no additional work is required here. I made the TheiaVariableContribution a bit easier to replaced (by binding it to self), but actual use cases for customization are unclear at this point, so I don't know how relevant this is.

How to test

• Start Theia with AI Chat view, and check that Theia Variables still work as expected (e.g. #lineNumber should appear in auto-completion)

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

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