Support PluginExt#extensionKind properly

[msujew]

What it does

Closes #8871

Adds a new constant isRemote that identifies whether the backend lives on a remote system. We reuse this info to give the plugin host the correct information about the extensionKind property.

How to test

1. Download and install this test plugin
2. Start Theia in browser mode and run the Display Extension Kind command. Assert that it shows Workspace.
3. Start Theia in Electron mode and run the same command. It shows UI.
4. From the Electron mode connect to a remote machine via SSH (or the dev-container feature). Running the command should show Workspace.

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

[rschnekenbu]

I could test with success on Theia example browser, electron and connected on remote using a dev container.
Code looks good also. Thanks @msujew!