plugin: re-organize proposed api files

[vince-fugnitto]

What it does

The pull-request re-organizes our theia-proposed.d.ts to follow the same conventions present in vscode, and also make it easier to update or add proposed apis in the future.

The changes:

• split the proposed apis by sections into their own files
• add the "extra" apis to theia-extra.d.ts properly

How to test

Review checklist

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

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[alvsan09]

I have reviewed the distribution of code to the different new files,
I found all code previously hosted in the former theia-proposed.d.ts file
mapped to the new files. 👍

The additions and modifications look good to me !

Testing was limited to start theia validate that no new activation failures arise,
and test some basic functionality.

[JonasHelming]

@rschnekenbu FYI

[rschnekenbu]

Same as @alvsan09, I did a quick test running theia and having no additional issues.
I have minor comments on textSearchQuery.d.ts and findTextInFile.d.ts proposals, which have duplicate code.
Otherwise, that looks very good to me and it's a great way to improve proposed APIs support!

[vince-fugnitto]

@rschnekenbu thank you for the feedback, I updated the pull-request accordingly.

[rschnekenbu]

@vince-fugnitto, that looks good to me!