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:

• 0.13 (nightly)
• 0.12
• 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
  • PreBranchCheckoustate:open t
  • 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.

[KirkEasterson]

Hi! I apologize for taking so long to sort out the merge conflict. I've updated the branch to fix those conflicts, but also removed the commits for the remaining events. This should make reviewing easier for you. Sorry for going overboard. I made a branch on my fork with those commits if you wish to revisit them (kirk/all-hooks).