feat: #1684 Ability to add custom popup switches

[AlexanderArvidsson]

Adds the ability to add custom argument switches to any popups by allowing users to create custom builder hooks to modify popups while they're building.

The above is implemented in config like this to provide the -o merge_request.create GitLab specific option:

require('neogit').setup({
  builders = {
    NeogitPushPopup = function(builder)
      --
      builder:switch('m', 'merge_request.create', 'Create merge request', { cli_prefix = '-o ', persisted = false })
    end,
  },
})

This PR also introduces the persisted = false option for switches, and it also migrates all existing non-persisted switches to this flag instead of the ignored_settings option. It doesn't remove ignored_settings, as people might have overridden it.
It also adds persisted = false to some extra popups that wasn't covered before (mostly force switches).

Fixes #1684.

Future changes

• Modify existing switches' options, to change their keybinds, text, persisted option, etc.
• Set an ordering for switches, or the ability to inject new options anywhere in the list.

[CKolkey]

Good stuff! A quick look at the code doesn't show any glaring issues. But would you mind adding some documentation to the help file? Under API or something that makes sense :)

[AlexanderArvidsson]

I added some docs under neogit_custom_popups and also updated the references to the default config, which has now changed (migrated to persisted flags, it shouldn't introduce any breaking changes afaict.

Let me know if you want to make some changes or move it somewhere else!