[plugin] withProgress should start task immediately

[AlexTugarev]

What it does

Fixes delayed task execution in withProgress. The resolution of the promise returned by the task may in some cases depend on the task being executed asap, e.g. the python extension expects that the task is running immediately in order to create a deferred promise, which if delayed breaks the removal of progress.

In case of python extension it led to the never ending Refreshing Python Interpreters progress.

How to test

1. install vscode extension for python and open a .py file
2. verify that Refreshing Python Interpreters will be hidden when done

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

[akosyakov]

tested that python extension does not live stale progress anymore