defer plugin contributions (preferences and tasks)

[akosyakov]

What it does

• fix register plugin contributions is slow in big workspaces #7673: with many VS Code extensions repetitive events are happening like update editor configurations and task schemas. This PR makes sure that such event defer changes in the same tick and perform them together.
• fix [plugin] fetching tasks is slow for big multi-root workspaces #7672: simplify $fetchTasks by removing filtering duplicate tasks. Testing showed that VS Code does not do it (see [plugin] fetching tasks is slow for big multi-root workspaces #7672 (comment)) and even after removing our implementation fails to return duplicate tasks. That's apparently an issue how the task extension treats the task identity: create tasks VS Code API compatibility tests #7681 (comment)

TODO:

• fix preference tests to check that there is not changes if schema set and unset in the same tick and there are if in different ticks
• make sure that update/replace configuration calls from the extension host process are not resolved before preference changes are propagated to the extension host

How to test

• check out https://github.com/akosyakov/jupyterlab/tree/ak/fetch_tasks

git clone https://github.com/akosyakov/jupyterlab.git \
cd jupyterlab/ \
git checkout ak/fetch_tasks

• start Theia for jlab.theia-workspace workspace

yarn start ../../../jupyterlab/jlab.theia-workspace

• open README.md as an editor and for preview
• enable CPU profiler and reload the page
• check that that registering plugin contributions is fast, not like here register plugin contributions is slow in big workspaces #7673 (comment)

Testing task fetching:

• install https://github.com/akosyakov/vscode-fetch-tasks-test

cd plugins/ \
git clone https://github.com/akosyakov/vscode-fetch-tasks-test.git \
cd vscode-fetch-tasks-test/ \
yarn && yarn compile

• restart Theia to load the plugin
• use Hello World to see all found tasks in output view
• you can also test according to add fetchTasks() and executeTask() to tasks API #6058 and use task definition as a reference to compare tasks #5975

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

[spoenemann]

check that that registering plugin contributions is fast

✅ handleContributions took only 327 ms.

use Hello World to see all found tasks in output view

✅ I see a long list of tasks, also in the Run Task dialog.

[RomanNikitenko]

@akosyakov

simplify $fetchTasks by removing filtering duplicate tasks

I played with your changes this morning and I think the filtering was related to customized tasks.
The difference is:

• before your changes it returned configured and detected tasks
• with your changes it returns configured, detected and customized tasks

To reproduce it with your plugin I modified a little your plugin to see whole configuration, not only name and source, then:

• use Hello World to see all found tasks in output view
• pay attention on number of tasks
• go to Terminal -> Configure task and configure a few tasks
• use Hello World to see all found tasks in output view
• pay attention on number of tasks

I don't know how vs code behaves for this case.
Sorry for the delay, maybe this comment will be useful for #7681.