Handle file system permissions

[msujew]

What it does

Closes #11509
Closes #199

This was more work than initially expected. It seems like we didn't handle readonly editors at all previously (see #199). So this change not only adds the file system permissions extension API, but also enables us to pass permission information all the way to the monaco model and phosphor tabbars.

How to test

1. Download and install this test extension
2. Run the 4 contributed Open file from file system commands. They should all result in read only editors except for the one that opens the mutable-in-mutable file.
3. Readonly editors should be marked as such (a lock icon) and should behave that way (i.e. the document is not editable).
4. Run the Check file system isWritableFileSystem command. Input different URI schemes which then displays a notification containing the result of the isWritableFileSystem call. The extension contributes the readonly and mutable file system schemes, which should return false/true respectively. Calls to file should also return true. Everything else should return undefined, unless contributed by an extension or plugin.

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]

The changes look good to me and work well 👍

• confirmed the commands contributed by the plugin work as expected
• confirmed that readonly editors display the lock decoration as a caption
• confirmed that readonly editors are readonly from a monaco standpoint (with the default message as a user attempts to edit the file)

[JonasHelming]

@colin-grant-work Do you want to have a look or can we merge this?

[colin-grant-work]

It looks reasonable to me, though I haven't tested it. I'm fine with merging.

[dpetroff]

Closes #199

I'm not sure it does. In theia 1.36, I set some files to read only on the file system with chmod, and the resource, filestat, etc. all still say isReadOnly=false