Write currently active env to prompt

[fmauch]

This will add the currently active environment to the shell prompt by default. E.g. if your prompt usually looks like this:

user@machine:~

it will look like this once the environment foobar is sourced:

[foobar] user@machine:~

It is written such that this behavior can be deactivated by setting the environment variable ROB_FOLDERS_DISABLE_PROMPT_MODIFICATION to a non-empty value.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 45.84%. Comparing base (112a2fd) to head (e5d04a4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #18 +/- ## ======================================= Coverage 45.84% 45.84% ======================================= Files 26 26 Lines 1444 1444 ======================================= Hits 662 662 Misses 782 782 ``` | [Flag](https://app.codecov.io/gh/fzi-forschungszentrum-informatik/robot_folders/pull/18/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fzi-forschungszentrum-informatik) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/fzi-forschungszentrum-informatik/robot_folders/pull/18/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fzi-forschungszentrum-informatik) | `45.84% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=fzi-forschungszentrum-informatik#carryforward-flags-in-the-pull-request-comment) to find out more.

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