Test: getEnvironmentVariableCollection API

[Tyriar]

Refs: #46696

• macOS @rebornix
• linux @octref
• windows @mjbvz

Complexity: 4

---

This new API allows extensions to contribute to an environment variable collection that enabled modifying environment variables on a process environment. Currently this is only used in the terminal but the naming is left generic so it could be used by other components.

To test, scaffold out an extension and try using the API. Here are some things to test:

• Read through the docs
• Try prepending, appending and replacing variables to existing variables and variables that doesn't exist
• Try running 2 extensions that both contribute to the same variable. These mutations are applied in the reverse order that they are registered, so if the first extension to activate does a Replace, no other extension mutations apply.
• Try think of any interesting usages outside of the common ones like prepending to PATH or setting language specific settings like JAVA_HOME and see if they also work with this model.

[rebornix]

Looks good in overall. My only confusion is persistent flag and how it works when reloading the window.

[mjbvz]

Works well overall.

The main issue I ran into is that I couldn't update the PATH on windows: #94069

I was also a little confused about ordering of updates, especially when working with multiple extensions. See #94081