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.

rockspec

@@ -0,0 +1,30 @@
+rockspec_format = '3.0'
+package = 'todo.nvim'
+version = 'scm-1'
+
+source = {
+  url = 'git+https://github.com/barrettruth/todo.nvim.git',
+}
+
+description = {
+  summary = 'Oil-like task management for Neovim',
+  homepage = 'https://github.com/barrettruth/todo.nvim',
+  license = 'MIT',
+}
+
+dependencies = {
+  'lua >= 5.1',
+}
+
+test_dependencies = {
+  'nlua',
+  'busted >= 2.1.1',
+}
+
+test = {
+  type = 'busted',
+}
+
+build = {
+  type = 'builtin',
+}