Resolves #59300

Until now only `$out/bin/weechat` and `$out/bin/weechat-headless` were installed
into the store path that will be used when running i.e. `nix-env -iA weechat`.

Further outputs like icons (`$out/share/icons`), man pages (`$man`) or the HTML
documentation (`$out/share/doc/weechat`) are omitted at the moment. As
this can be fairly confusing I figured that it's better to copy those
files into the environment as well.

As `buildEnv` doesn't appear to support output splitting (you can only
install additional outputs of `paths` using `extraOutputsToInstall`),
it's easier for now to always install the `man` output by
default.

Man page installation can be turned off like this now:

```
weechat.override { installManPages = false; }
```

weechat: install all outputs into the final store path