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.

.luarc.json

@@ -0,0 +1,8 @@
+{
+  "runtime.version": "LuaJIT",
+  "runtime.path": ["lua/?.lua", "lua/?/init.lua"],
+  "diagnostics.globals": ["vim", "jit"],
+  "workspace.library": ["$VIMRUNTIME/lua", "${3rd}/luv/library"],
+  "workspace.checkThirdParty": false,
+  "completion.callSnippet": "Replace"
+}