Add a `manual` detection mode

[sellout]

I want a way for Auto-Dark to manage the pairs of themes & hooks, but to not do any auto-detection of system mode changes.

This is useful for testing, but also for niche systems that don’t have any of the existing systems. This is already possible, but some changes to make it more ergonomic:

• an auto-dark-toggle-appearance command, so no internal functions need to be called; and
• a way to get auto-dark-mode to not try to re-determine the detection method (either adding a manual detection method, or moving the auto-dark--determine-detection-method call to the defcustom form and treating nil as the manual mode)[^1].

[^1]: I prefer using nil for manual mode (read as “there is no detection method”), but perhaps it’s desirable to have a case where re-enabling the theme tries to re-detect the method (as it does now). I think I would prefer having a public auto-dark-determine-detection-method command that users can explicitly call if they want to re-detect.

[sellout]

Ah, and manual mode should also not run the timer.