Stricter launchd -> StartCalendarInterval type:

- Verify that the integers passed to `Minute`, `Hour`, etc. are within
  range.

- When provided, the value for StartCalendarInterval must be a non-empty
  list of calendar intervals and must not contain duplicates entries
  (throw an error otherwise).

- For increased flexibility and backwards-compatibility, allow an
  attrset to be passed as well (which will be type-checked and is
  functionally equivalent to passing a singleton list). Allowing an
  attrset or list is precisely in-line with what `launchd.plist(5)`
  accepts for StartCalendarInterval.

Migrate `nix.gc.interval` and `nix.optimise.interval` over to use this
new type, and update their defaults to run weekly instead of daily.

Create `modules/launchd/types.nix` file for easier/modular use of
launchd types needed in multiple files.

Documentation:

- Update and improve wording/documentation of launchd's
  `StartCalendarInterval`.

- Improve wording/documentation of `nix.gc.interval` and
  `nix.optimise.interval` ("time interval" can be misleading as it's
  actually a "calendar interval"; e.g. `{ Hour = 3; Minute = 15;}`
  runs daily, not every 3.25 hours).

This process was automated by [my fork of `nix-doc-munge`]; thanks
to @pennae for writing this tool! It automatically checks that the
resulting documentation doesn't change, although my fork loosens
this a little to ignore some irrelevant whitespace and typographical
differences.

As of this commit there is no DocBook remaining in the options
documentation.

You can play along at home if you want to reproduce this commit:

    $ NIX_PATH=nixpkgs=flake:nixpkgs/c1bca7fe84c646cfd4ebf3482c0e6317a0b13f22 \
      nix shell nixpkgs#coreutils \
      -c find . -name '*.nix' \
      -exec nix run github:emilazy/nix-doc-munge/0a7190f600027bf7baf6cb7139e4d69ac2f51062 \
      {} +

[my fork of `nix-doc-munge`]: https://github.com/emilazy/nix-doc-munge

The nix.useDaemon option enables using a multi-user install without
managing the nix installation with nix-darwin.  The following checks
apply to both cases.

This configures the user that runs the garbage collector, particularly
useful for single-user installs.  Otherwise it runs as root resulting in
permission issues afterwards.