Export VIRTUAL_ENV_PROMPT in activation scripts (#2194)

[jimporter]

See issue #2194. This change exports a VIRTUAL_ENV_PROMPT environment variable during venv activation so that users can use this in their own custom shell prompts if they like. I didn't add a test for this since I don't see any similar tests, and I'm not quite sure the bet way to go about writing one...

I manually tested these changes in Bash and the results look good to me, though my changes for the other shell scripts are speculative. I did my best to guess how they should work, but there may be bugs!

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[gaborbernat]

[gaborbernat]

Please add extend the test suite to test for this.

[jimporter]

Please add extend the test suite to test for this.

I'll take a look. I'm going to mark this as a draft for now until I get tests resolved (and ideally for my second venv PR to merge so that I can be sure the behavior is the same between venv and virtualenv).

[jimporter]

@gaborbernat Ok, hopefully these tests pass. I had to make some educated guesses for a few of the shells.

[gaborbernat]

You'll need to make the CI pass 😊

[jimporter]

You'll need to make the CI pass 😊

Will do. I'm working on fixing them on my fork right now (but in a separate branch to keep the noise of pushing to CI repeatedly down over here). I'll push to this branch once things look green.

[jimporter]

@gaborbernat Ok, I think these tests should be good now. One small note is that I renamed VIRTUAL_PROMPT to VIRTUAL_PREFIX in Nushell to reduce ambiguity, though I could revert that. As far as I can tell, that variable is only there to make it easier to write a unit test (this is the variable that holds (<env-name>) , as opposed to VIRTUAL_ENV_PROMPT which holds <env-name>).

[gaborbernat]