build: add luarocks packaging and bump stylua (#19)

Problem: oil.nvim had no luarocks rockspec, so users of rocks.nvim
and similar tools could not install it from the registry. The stylua
CI action was also pinned to an older version.

Solution: add scm-1 rockspec and a luarocks publish workflow that
gates on tests passing before publishing on version tags. Bump
stylua action from v2.0.2 to v2.1.0.

Closes: barrettruth/oil.nvim#14

oil.nvim-scm-1.rockspec

@@ -0,0 +1,21 @@
+rockspec_format = '3.0'
+package = 'oil.nvim'
+version = 'scm-1'
+
+source = {
+  url = 'git+https://github.com/barrettruth/oil.nvim.git',
+}
+
+description = {
+  summary = 'Neovim file explorer: edit your filesystem like a buffer',
+  homepage = 'https://github.com/barrettruth/oil.nvim',
+  license = 'MIT',
+}
+
+dependencies = {
+  'lua >= 5.1',
+}
+
+build = {
+  type = 'builtin',
+}