feat(core): new updater with stable and nightly channels

[mehalter]

This PR adds a fully fledged updater that will handle stable vs. nightly updates.

• If you follow stable then AstroNvim will only update to tagged versions and will also use provided Packer snapshots to automatically manage the versions of the core provided plugins. This will guarantee that while on the stable channel these plugins don't provide breaking updates that break the core AstroNvim functionality.
• If you follow nightly this will update to the latest commits on the specified branch

This will have a new configuration section in the user/init.lua file that looks something like the following:

return {
  updater = {
    remote = "origin", -- remote to use
    channel = "nightly", -- "stable" or "nightly"
    version = "latest", -- "latest", tag name, or regex search like "v1.*" to only do updates before v2 (STABLE ONLY)
    branch = "main", -- branch name (NIGHTLY ONLY)
    commit = nil, -- commit hash (NIGHTLY ONLY)
    pin_plugins = true, -- true, false, or a string for a specific AstroNvim snapshot to use (true will only track the current version if channel is "stable")
    skip_prompts = false, -- skip prompts about breaking changes
    show_changelog = true, -- show the changelog after performing an update
    remotes = { -- easily add new remotes to track
      ["remote_name"] = "https://remote_url.come/repo.git", -- full remote url
      ["remote2"] = "github_user/repo" -- GitHub user/repo shortcut,
      ["remote3"] = "github_user", -- GitHub user assume AstroNvim fork
    },
  },
}

This will allow users to also pin to a specific stable version and on nightly pin to a specific branch and/or commit. We are also planning on providing automatic checking if there are breaking changes and prompt the user if they want to continue and also have the option to print the changelog after an update is successfully performed.

TODO

• Calculate update path for both stable or nightly paths with possible specified version/branch/commit
• Check for breaking changes and prompt user if they want to abort
• Check for conflicting user modifications to the base installation and prompt if they want to abort before we reset
• Perform update
• Use checkout when user specifies commit on nightly
• Load Packer snapshot if on stable
• Display commit log and prompt to restart
• Add custom remotes + remote specification to make testing PRs or forks easier
• Make everything prettier with the vim.api.nvim_echo API