build: add tooling and dev environment configs

Problem: need formatting, linting, testing, and nix dev shell configs.

Solution: add stylua, selene, lua-language-server, busted, pre-commit,
flake.nix, and luarocks rockspec configuration.

stylua.toml

@@ -0,0 +1,8 @@
+column_width = 100
+line_endings = "Unix"
+indent_type = "Spaces"
+indent_width = 2
+quote_style = "AutoPreferSingle"
+call_parentheses = "Always"
+[sort_requires]
+enabled = true