Resolve command variables contributed by debuggers

[alvsan09]

What it does

Fixes: #10710, Fixes: #2819

The debuggers contribution point provides the possibility to specify
command id variables which can therefore be referenced within debug
configurations.

This change reads these contributed variables and feeds them to the
variable resolver so they can be used just before resolving values
via commands.

See "variables" at:
https://code.visualstudio.com/api/references/contribution-points#contributes.debuggers

In addition, the 'VariableResolverOptions' includes an option to
indicate if all variables were successfully resolved, this enables
to stop the processing and prevent the possibility of error
notifications e.g. in case a user did not select a valid option.

How to test

The following test uses the command "PickProcess" contributed by the node-debug extension.
We will need to start a node application that we can attach to i.e. using the --inspect option.

1. You can use the following test application, created for this purpose:
   https://github.com/alvsan09/hello_express.git

2. Build the application by running

yarn

3. Run the application

node --inspect hello_express.js

4. Open a browser pointing to localhost:3099, make sure you see the Hello world! message and a counter incrementing every refresh.
5. Open the folder holding your application with theia
6. From the debug view, start the provided configuration i.e. Attach to node process,
   NOTE: This configuration uses the following variable that needs to be resolved "${command:PickProcess}"
7. From the list of node processes select the one hello_express.js
   NOTE: The fact that a list of node processes is presented for selection is an indication of the successful resolution of the command variable mentioned in the previous step.
8. Open the file hello_express.js in editor and place a break point inside the get block. The break point shall become active.
9. In the browser, Refresh the application and make sure the break point is hit.
10. Repeat steps 1-6 above but don't select any of the node processes e.g. pressing escape or clicking outside of the options area.
11. Note that no debug session is started and no errors are reported.

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

[msujew]

I can confirm that the change allows to resolve command variables 👍

• Variables such as ${command:PickProcess} are resolved correctly
  • Selecting such a debug config opens the quick input to select a process id
  • Afterwards, the debugger runs as expected
• Other debug configs continue to work as expected

I have some discussions on the code that I'd like to see resolved before approving.

[alvsan09]

I have some discussions on the code that I'd like to see resolved before approving.

@msujew, please take a look to the recent updates which address your remarks,
Thanks !!

[msujew]

LGTM!