discord: combine nixcord, vencord, vesktop

[Flameopathic]

combines the nixcord, vencord, and vesktop hm.nix modules into one.

handles the config separately as the nixcord module requires odd conditions for evaluation.

unsure how to handle the testbeds.

[trueNAHO]

unsure how to handle the testbeds.

@danth, /modules/firefox/ suffers from the same problem of having one testbed per module, although multiple testbeds would be beneficial.

What do you think of optional /modules/<MODULE>/testbeds/<TESTBED>.nix testbeds, while asserting that /modules/<MODULE>/testbed.nix should not exist while any /modules/<MODULE>/testbeds/<TESTBED>.nix exists. However, this prevents the use of a highly convenient local /modules/<MODULE>/testbeds/common.nix abstraction. Either, we handle certain file names differently (probably a bad idea) or such files are expected to go to /modules/<MODULE>/common.nix. Since we are looking for specific files and directories inside /modules/<MODULE>/, adding arbitrary named files in this scope does not cause problems.

Note that implementing this functionality should probably not fall in the scope of this PR. It should be fine to keep only one testbed in this PR and add the others back once this functionality is implemented.

[danth]

@trueNAHO That sounds reasonable. Ignoring specific file names would be confusing and limiting, so I agree, it's better to just import from the parent directory when some shared code is needed.