ci: package the plugin for lua(rocks)

 ### Summary

 This PR is part of a push to get (neo)vim plugins on [LuaRocks](https://luarocks.org/labels/neovim).

     * See [this blog post](https://mrcjkb.github.io/posts/2023-01-10-luarocks-tag-release.html), which follows up on [a series of posts](https://teto.github.io/posts/2021-09-17-neovim-plugin-luarocks.html) by @teto.

     * I'm aware this is a vimscript plugin. But that does not prevent `luarocks` from being able to package this smile
       In fact, as a popular plugin that many other plugins depend on, having `vim-repeat` on LuaRocks.org
       could be great for further adoption.

     * If you support this, please let me know if you would like me to add equivalent PRs to your other plugins.

 ### Things done:

     * Add a workflow that publishes tags to LuaRocks when a tag is pushed.

 ### Notes

     * Tagged releases are installed locally and then published to LuaRocks.org.
       On PRs, a test release is installed, but not published.

     * For the release workflow to work, someone with a LuaRocks.org account will have to add their [API key](https://luarocks.org/settings/api-keys) to this repo's [GitHub actions secrets](https://docs.github.com/en/actions/security-guides/encrypted-secrets#creating-encrypted-secrets-for-a-repository).

     * Due to a shortcoming in LuaRocks ([label doesn't get picked up from rockspec luarocks/luarocks-site#188](luarocks/luarocks-site#188)), the `neovim` and/or `vim` labels have to be added  to the LuaRocks package manually (after the first upload), for this plugin to show up in [luarocks.org/labels/neovim](https://luarocks.org/labels/neovim) or [luarocks.org/labels/vim](https://luarocks.org/labels/vim), respectively.

 I have tested this with the [`quick-scope`](https://github.com/unblevable/quick-scope) plugin in Neovim using the following minimal config:

 ```
 mkdir /tmp/nvim-test
 luarocks install --tree /tmp/nvim-test quick-scope
 cd /tmp/nvim-test
 nvim minimal.lua # paste minimal.lua below
 nvim -u minimal.lua test.txt
 ```

 ```lua
 -- minimal.lua

 -- Ignore default config
 local config_path = vim.fn.stdpath('config')
 vim.opt.rtp:remove(config_path)

 -- Ignore default plugins
 local data_path = vim.fn.stdpath('data')
 local pack_path = data_path .. '/site'
 vim.opt.packpath:remove(pack_path)

 vim.opt.runtimepath:append('lib/luarocks/rocks-5.1/quick-scope/scm-1/')
 ```

 ...and it works tada

 ![image](https://user-images.githubusercontent.com/12857160/258669184-ea7bd712-6227-486f-ba21-d9ba44de910c.png)

 **Adding the API key (screen shot)** ![github-add-luarocks-api-key](https://user-images.githubusercontent.com/12857160/211071297-afb129be-7a8f-4662-b282-9d52bb0286de.png)

.github/workflows/release.yml

@@ -0,0 +1,21 @@
+name: "release"
+on:
+  push:
+    tags:
+      - 'v*'
+  pull_request:
+jobs:
+  luarocks-upload:
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v3
+      - name: LuaRocks Upload
+        uses: nvim-neorocks/luarocks-tag-release@v5
+        env:
+          LUAROCKS_API_KEY: ${{ secrets.LUAROCKS_API_KEY }}
+        with:
+          detailed_description: |
+            Find the enemy and replace them with dark power
+          dependencies: |
+            plenary.nvim
+          license: "mit"