Correctly expose extensionUri for frontend modules

[thegecko]

What it does

Depends on #10721.

The extensionContext.extensionUri was incorrectly returning a file schemed Uri for frontend modules.
This meant loading assets based on context.extensionUri wasn't possible (and breaks some loading in VS Code web extensions)

This PR ensures extensionUri is correctly formed based on the type of module.

It also updates the pluginContext to use the underlying plugin extensionPath and extensionUri fields rather than recalculating them.

How to test

Create a frontend module (VS Code web extension or Theia frontend plugin) and inspect the context.extensionUri;

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]

@thegecko do you mind providing additional information or perhaps a plugin to be able to test the feature, the same is true for #10745. I tried making a vscode web extension but it did not seem to work.

[thegecko]

VSCode Web Extensions won't work until this is merged: #10721

There's a link there to a sample extension you can use.

To test this, cherry-pick that fix and use the example VS Code extension (adding extensionUri and joinPath usage).

However, it may be easier to create a Theia Plugin which uses extensionUri and joinPath

[vince-fugnitto]

The changes look good to me, I see the following:

master:

pull-request:

I also confirmed that non-web extensions work like today.