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.

.busted

@@ -0,0 +1,9 @@
+return {
+  _all = {
+    lua = 'nvim -l',
+    ROOT = { './spec/' },
+  },
+  default = {
+    verbose = true,
+  },
+}