ensure that plugin tasks are registered before accessing them

[akosyakov]

What it does

fix #5861 - fire onCommand:workbench.action.tasks.runTask in order to activate VS Code extensions and register task providers

How to test

See #5861 (comment):

• install built-in npm vscode extension: https://registry.npmjs.org/@theia/vscode-builtin-npm/-/vscode-builtin-npm-0.2.1.tgz
• open a workspace with npm scripts defined in package.json
• try to run a task from the quick task palette without opening package.json
• check that npm scripts are detected as tasks

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 tested using the npm vscode-builtin-extension and can confirm that I now see
the tasks when executing the command Run Tasks....

The problem reported from #5861 is fixed.

[akosyakov]

I will let this PR sit till tomorrow if someone wants to have a look.