preferences: add json commands

[vince-fugnitto]

What it does

The pull-request adds the following commands:

• open user preferences (json)
• open workspace preferences (json)
• open folder preferences (json)

The folder command will prompt users to first select a root before opening.

How to test

1. in a single-root workspace execute open user preferences (json) - the user settings.json should be opened in an editor.
2. in a single-root workspace execute open workspace preferences (json) - the workspace settings.json should be opened in an editor.
3. in a multi-root workspace with multiple folders execute open folder preferences (json) - the respective settings.json should be opened after selecting a root.
4. confirm that open folder preferences (json) does not appear in a single-root workspace or a multi-root workspace without any root folders.

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

Signed-off-by: vince-fugnitto vincent.fugnitto@ericsson.com

[msujew]

That already looks really good. I have a few questions/points of discussion before approving though.

[msujew]

As far as I can see there's no normal Open Folder Preferences command in Theia right now. Perhaps it is out of scope for this PR, but do you mind adding it? The appropriate frontend UI in the preferences view is already supported for that:

[colin-grant-work]

Looks good to me. I confirm the new commands appear and work when they're supposed to, and do not appear when they should not. They all have the ID's that a VSCode plugin would expect.

[vince-fugnitto]

@msujew I'll merge and @colin-grant-work mentioned a follow-up pull-request to add the missing command (out of scope for this pull-request) with some possible additional refactorings.