feat(luatest): create a self-contained test runner

Adds a new scripts/run-luatest.sh script to provide a consistent,
self-contained way to run project unit tests, including locally.

The script handles:

 - Checking for necessary tools (rg, ag)
 - Cloning or updating plenary.nvim into the target/tests/deps directory

The Makefile is updated to use this new script, making make luatest the
single entry point for running unit tests.

The lua.yaml workflow is simplified to use this new make target,
ensuring the CI environment uses the exact same testing logic.

.github/workflows/lua.yaml

@@ -48,8 +48,6 @@ jobs:
         run: |
           export PATH="${PWD}/_neovim/bin:${PATH}"
           export VIM="${PWD}/_neovim/share/nvim/runtime"
-          # install nvim-lua/plenary.nvim
-          git clone --depth 1 https://github.com/nvim-lua/plenary.nvim ~/.local/share/nvim/site/pack/vendor/start/plenary.nvim
           nvim --version
           make luatest