[needs websockets] Add development flake file

[PartyWumpus]

Please tick as appropriate:

• I have tested this code on a steam deck or on a PC
• My changes generate no new errors/warnings
• This is a bugfix/hotfix
• This is a new feature

If you're wanting to update a translation or add a new one, please use the weblate page: https://weblate.werwolv.net/projects/decky/

Description

Adds a flake.nix file so developers can get all relevant dependencies by just typing nix develop.
Uses poetry2nix so all python deps will get installed at the correct versions without needing to update the flake.
It's currently set up to build each python library from source instead of using the wheel but this can be changed if preferable.
Possibly an issue is that the env is not automatically updated when new deps are added, it must be restarted.
Also the way pyright is working is a bad solution.

[AAGaming00]

LGTM, could probably add a .envrc too for direnv