fix user storage regressions

[akosyakov]

What it does

• fix bug: error thrown when changing themes without a 'settings.json' #8302: ensure that user storage URI has a top-level directory. Otherwise FileService fails to create the user directory.
• remove hardcoded constants from preferences UI. It is a secret of the preference service to know which config URI should be use for each scope in the context of the given resource.
• fix keymaps: 'Open Keyboard Shortcuts in JSON' command should create file if it does not exist #8301: use monaco model to handle keymaps. It allows to check whether model is invalid before opening it, in addition keymaps are now updated with minimal edits and preserving user formatting and comments.

How to test

• Start theia app and remove .theia in user home. Try to open keymaps and user settings.
• Try to modify keymaps and check that changes are reflected.
• Try to open settings file for different scopes, i.e. user, workspace, folder as a workspace and workspace folder.

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

[vince-fugnitto]

I verified and the regression works successfully! 👍

I verified the following (removing the ~/.theia before each test):

• changing themes works correctly (the settings.json is successfully created)
• opening the keymaps.json works successfully (the file is created if it does not exist)
• modifying the keymaps.json through the UI works successfully again

[akosyakov]

there is no