feat(setup)!: remove ensure_install field

Instead, call `require('nvim-treesitter').install( { ... } )` manually.

This gives users full control over how they want to install parsers
(sync, from grammar, limited concurrency) and obviates the need for
calling `setup` for most users.

.luarc.json

@@ -5,12 +5,13 @@
   },
   "workspace": {
     "library": [
-      "lua",
       "$VIMRUNTIME",
-      "${3rd}/luv/library",
       "${3rd}/busted/library"
     ],
-    "checkThirdParty": false
+    "ignoreDir": [
+      "tests"
+    ],
+    "checkThirdParty": "Disable"
   },
   "diagnostics": {
     "groupFileStatus": {