Improve venv shell activation in fish shell

[MrGreenTea]

Instead of mimicking a user typing the command we use fish's builtin --init-command option. This is similar to how poetry already does it in nushell. This makes the experience a bit nicer because instead of it looking something like this:

poetry on  master is 📦 v1.8.0.dev0 via 🐍 v3.12.0 
❯ poetry shell
Spawning shell within ~/virtualenvs/poetry-4gAzItFB-py3.11
Welcome to fish, the friendly interactive shell
Type help for instructions on how to use fish

poetry on  master is 📦 v1.8.0.dev0 via 🐍 v3.12.0 
❯ source ~/.cache/pypoetry/virtualenvs/poetry-4gAzItFB-py3.11/bin/activate.fish

poetry on  master is 📦 v1.8.0.dev0 via 🐍 v3.12.0 (poetry-py3.11) 
❯  _

it now looks like this (notice the missing source invokation)

❯ poetry shell
Spawning shell within ~/virtualenvs/poetry-4gAzItFB-py3.11
Welcome to fish, the friendly interactive shell
Type help for instructions on how to use fish

poetry on  master is 📦 v1.8.0.dev0 via 🐍 v3.12.0 (poetry-py3.11) 
❯  _

The --init-command option has been released with fish version 2.7b1 in 2017 according to their changelog so in my opinion should not cause compatibility issues.

I didn't find any tests for the nu behavior but would add tests for this if wanted and with some small pointers about how you would like this to be tested.

• Added tests for changed code.
• Updated documentation for changed code.

[radoering]

LGTM. Very good PR description. 👍

The --init-command option has been released with fish version 2.7b1 in 2017 according to their changelog so in my opinion should not cause compatibility issues.

Thanks for pointing this out. Should be fine.

I didn't find any tests for the nu behavior but would add tests for this if wanted and with some small pointers about how you would like this to be tested.

Nobody has come up with a sensible way to test this yet. Probably, simple tests would be quite pointless and useful tests are just too much effort.

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