Implement scope API on env var collections

[tsmaeder]

What it does

Implements the EnvironmentVariableScope on the GlobalEnvironmentVariableCollectoin class in the VS Code API.

Fixes #12940

Contributed on behalf of STMicroelectronics

How to test

1. Install the attached plugin.
2. Open a workspace with two roots
3. Start a terminal, chose any root as the cwd
4. Print out the environment variables. Note that there is no variable called ROOT_MUTATED.
5. Run the command "Hello World". It activates the extensions
6. Open a terminal on the first workspace root
7. Print out the environment variables, not that there are not the env vars ROOT_MUTATED, PERSISTENT_MUTATED and MUTATED2 present
8. Open a terminal on the second workspace roo
9. Print out the environment variables. Note that there is only the variable called ROOT_MUTATED present.

Follow-ups

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]

Here is the extension:
envscopetest-0.0.1.zip

And here's the source:
envscopetest.zip

[tortmayr]

Code looks good to me and the API is now inline with its VS Code Counterpart.
Tested with the extension as described and everything works as expected (Ubuntu 22.04).

There are some merge conflicts that need to be resolved but other than that LGTM 👍🏼

[tsmaeder]

@tortmayr I rebased the branch

[tortmayr]

Retested and works as expected!