Localize plugins using language packs

[msujew]

What it does

Addresses one of the open issues of #9538: Enables Theia to localize vscode extensions/plugins using language packs.

Basically addresses two problems regarding vscode extensions:

First off, it changes how the plugin package is read. Placeholders are now replaced when the frontend loads and not when the plugin is deployed.

Secondly, through reverse engineering of the vscode-nls module, we are able to create translationConfig files that point to the correct localization file for each vscode extension. These files are cached in the localization-cache folder in the Theia home directory.

How to test

1. Install the vscode.npm builtin extension and a language pack of your liking.
2. Verify that the npm Scripts view is visible and in english.
3. Open a package.json file that contains a scripts section. Hover over one of the scripts and observe the Run Script popup:
   
4. Use the Configure Display Language command to change to a different language.
5. The application reloads and the npm Scripts view should now have a different name. (Tests that the package.json is still correctly translated)
6. Hover over one of the scripts from the package.json file and observe a changed popup. (Tests that the plugin host runtime is using the correct localization)
   

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]

@msujew one of the problems I see is that we're assuming that plugins are deployed at the back end, not the plugin host. I think this prevents us from doing stuff like remote workspaces, as VS Code does. I think just using the packageUri field instead of packagePath and using the FileService instead of node file system should get us there.

[tsmaeder]

Also: could you open an epic with problems you know that need solving with NLS? Would be good to be able to track the state of the implementation.

[msujew]

Could you open an epic with problems you know that need solving with NLS?

Sure, done #10188

One of the problems I see is that we're assuming that plugins are deployed at the back end, not the plugin host.

I'm not sure I can follow. Can the plugin host be deployed to other resources (which do not have access to the file system of the backend) using remote workspaces? The issue is that the extensions/plugins internally try (using the vscode-nls package) to read the translation files using the same file system where the language pack was installed.

[jbicker]

Works as described! Great work!

[msujew]

@tsmaeder FYI, I will come back to the remote plugin host issue later, in another PR. I'll merge this for now.