feat(treesitter): add vim.treesitter.start(), enable for Lua

* Add vim.treesitter.start() for starting treesitter highlighting via
  ftplugin or autocommand (can be extended later for fold, indent,
  matchpairs, ...)
* Add vim.treesitter.stop() for manually stopping treesitter
  highlighting
* Enable treesitter highlighting for Lua if
  `vim.g.ts_highlight_lua = true` is set in `init.lua`

runtime/ftplugin/lua.lua

@@ -0,0 +1,3 @@
+if vim.g.ts_highlight_lua then
+  vim.treesitter.start()
+end