Add "pre-event" hooks

[KirkEasterson]

This is a POC for hooks as requested in #1909. This PR will remain a draft during the iteration and planning phase. Once an approach is agreed upon, then this PR will be cleaned up to be a candidate for merging.

I am not fluent in lua, so please be critical with coding standards.

Testing done:
I've run this locally on my own config to solve the problem from the aforementioned issue. I added the following to my neogit config:

    hooks = {
      ["PreBranchCheckout"] = function()
        local neogit = require("neogit")
        local autosession = require("auto-session")

        neogit.close()
        autosession.SaveSession(nil, false)
        neogit.open()
      end,
    },

The function body can be replaced with print("Running PreBranchCheckout") for a simple verification.

Neovim versions tested on:

• 0.12 (nightly)
• 0.11
• 0.10
• 0.9

OSes tests on:

• arch linux
• macos
• windows

Remaining work:

• add more hooks, ideally one-to-one for all events
  • PreStatusRefreshed
  • PreCommitComplete
  • PrePushComplete
  • PrePullComplete
  • PreFetchComplete
  • PreBranchCreate
  • PreBranchDelete
  • PreBranchCheckout
  • PreBranchReset
  • PreBranchRename
  • PreRebase
  • PreReset
  • PreTagCreate
  • PreTagDelete
  • PreCherryPick
  • PreMerge
  • PreStash
  • RefsRefreshed
  • DiffLoaded
  • Bisect
  • WorktreeCreate
• update documentation
• update tests

[CKolkey]

That's about what I had in mind too. I would, though, create a "hook.lua" file with a run_hook("...", {}) function that can wrap the conditional logic in one place. If there's no hook, it's just a no-op.

The second arg can be an optional table of arguments to pass into the hook function.

[KirkEasterson]

Changes since last push:

• rebase on latest master
• create hook.lua file with your recommendations
• rename NeogitHooks alias to NeogitHook

[KirkEasterson]

I implemented the remaining events as hooks. The commits can be squashed later. I assumed separate commits would be easier to review.

[KirkEasterson]

I wasn't certain about where to call some hooks. Some calls can be moved a few lines while others can be moved to different functions.

[KirkEasterson]

I've updated the PR with documentation.