Open new, multi-root workspace from CLI

[colin-grant-work]

What it does

This PR implements the ability to open a multi-root workspace by entering multiple folders on the command line.

How to test

1. Run the Theia CLI and enter multiple folders on the command line.
2. Depending on the number of (existing) folders entered

• 0 -> open last workspace
• 1 -> open folder as workspace (or open workspace file)
• > 1 -> create new untitled workspace file and open it.

Decisions:

• If multiple entries are found, they're filtered down to existing folders; non-folders are ignored.
• If a single entry is found, it's handled fairly naively: the rest of the workspace system knows to ignore non-existent results from the workspace CLI
• There's some URI logic that could use @msujew's advice.

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 confirmed that the following works well for me on linux:

• yarn electron start without a workspace will open the previously opened workspace
• yarn electron start ${folder} will open the application with the folder as a workspace
• yarn electron start ${invalid-folder} will open the application without a workspace
• yarn electron start ${folder-1} ${folder-2} will open the application with a multi-root workspace with the supplied folder roots
• yarn electron start ${folder-1} ${folder-2} ${invalid-folder} will open the application with a multi-root workspace with the supplied folder roots omitting the invalid one

[colin-grant-work]

@msujew, would you be willing to weigh in on the URI handling in this PR?

[msujew]

Noticed some issues on Windows with this. I checked that it works correctly with the suggestion applied 👍

You will need to rebase in order to use Path.fsPath().

[colin-grant-work]

@msujew, many thanks! 🙌

[msujew]

@colin-grant-work You're welcome :)

After the latest changes everything works well for me!