feat: add on_create, on_delete, and on_switch lifecycle hooks

[Stan15]

Summary

Add lifecycle hooks: on_create, on_delete, on_switch

Adds optional callback hooks to the setup config that fire after worktree lifecycle events. This is very for automating environment setup that can't be shared across worktrees - e.g. symlinking .env files, running build scripts, e.t.c.

Usage:

require("worktrees").setup({
    on_create = function(path)
        -- called after a worktree is created
    end,
    on_delete = function(path)
        -- called after a worktree is deleted
    end,
    on_switch = function(from_path, to_path)
        -- called after switching worktrees
    end,
})

Details:

• All hooks are optional (nil by default)
• on_switch receives the path switched from and the path switched to
• When a newly created worktree is auto-switched to, on_create fires before on_switch
• Documented in both README and vimdoc

[afonsofrancof]

Hi!
Thanks for contributing!
I like this idea very much.
One small thing though. I would prefer if "from_path" actually returned nil if the current worktree root isn't found, instead of "(root)".
This signifies that we didn't switch from a worktree, but from somewhere else.
Other than that LGTM.

[Stan15]

Thanks for the feedback, @afonsofrancof! I can make that change and push out an update in a sec 👍