chore: add default luarc.json for Lua development (#16487)

The Sumneko Lua language server has matured quite a bit and many
Neovim developers use it while working on Neovim. Having a default
configuration for Neovim development is a nice convenience (and
dovetails well with the auto-generated compile_command.json for C
development).

The file is shipped under `contrib` and users can make use of it by
symlinking to `.luarc.json` in the project root.

contrib/luarc.json

@@ -0,0 +1,23 @@
+{
+    "runtime.version": "LuaJIT",
+    "diagnostics": {
+        "enable": true,
+        "globals": [
+          "vim",
+          "describe",
+          "it",
+          "before_each",
+          "after_each",
+          "setup",
+          "teardown"
+        ]
+    },
+    "workspace": {
+        "library": {
+            "runtime/lua": true
+        },
+        "maxPreload": 2000,
+        "preloadFileSize": 1000
+    },
+    "telemetry.enable": false
+}