Refactor Code for Terminal Environment Variable Experiment

[anthonykim1]

We should refactor: src/client/terminals/envCollectionActivation/service.ts in a way that we create two separate service for the new terminal environment variable experiment:

One service that would perform fetching environment variables via process (fallback option in case shell integration way doesn't work), another serivce that would fetch environment variables using shell integration enabled hidden terminal approach.

[anthonykim1]

No longer needed, getting rid of the experiment: #24567