feat(plugin-ext): Fix instanceof check to handle VSCode URIs

[t1m0thyj]

What it does

Fixes #13949

How to test

Follow the steps to reproduce in the issue and verify that it's fixed.

Follow-ups

There are more places in the plugin-ext package that have an instanceof check for Theia URIs. I have restricted my changes to the scope of the file plugin-context.ts.

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

[tsmaeder]

Hi @t1m0thyj, thanks for the PR. I'm afraid I'll have to disagree with my esteemed colleague @msujew: the instanceof check is for the API-class that functions as the declared (theia|vscode).URI class and I think it's correct. Plugins must pass in an instance of that class when invoking showTextDocument(). That class is currently based on the VS Code-internal URI class, but that is purely accidental. We already have a mechanism for converting command arguments to their proper types: CommandRegistryImpl.registerArgumentProcessor(). We cold implement an argument processor that converts VS Code-internal URI's to proper API URIs and register it in the constructor. VS Code extensions can never check for the VS Code-internal class, so we're safe there. The only way we could have breakage is if a registered command would invoke a built-in command with one of it's arguments, but with the current implementation based on the VS Code URI, it would still work.

[tsmaeder]

@t1m0thyj btw: in order to contribute to this project, you'll have to sign the committer agreement with the Eclipse foundation for the email address used to sign the commits. There's a link to the signature page on the right column here: https://api.eclipse.org/git/eca/status/gh/eclipse-theia/theia/13961