chore: convert .luarc.json to nested format and add busted library

Problem: .luarc.json used the flat dotted-key format which is not the
canonical LuaLS schema. The busted library was also missing, so LuaLS
could not resolve types in test files.

Solution: rewrite .luarc.json using nested objects and add
${3rd}/busted/library to workspace.library.

.luarc.json

@@ -1,8 +1,16 @@
 {
-  "runtime.version": "LuaJIT",
-  "runtime.path": ["lua/?.lua", "lua/?/init.lua"],
-  "diagnostics.globals": ["vim"],
-  "workspace.library": ["$VIMRUNTIME/lua", "${3rd}/luv/library"],
-  "workspace.checkThirdParty": false,
-  "completion.callSnippet": "Replace"
+  "runtime": {
+    "version": "LuaJIT",
+    "path": ["lua/?.lua", "lua/?/init.lua"]
+  },
+  "diagnostics": {
+    "globals": ["vim"]
+  },
+  "workspace": {
+    "library": ["$VIMRUNTIME/lua", "${3rd}/luv/library", "${3rd}/busted/library"],
+    "checkThirdParty": false
+  },
+  "completion": {
+    "callSnippet": "Replace"
+  }
 }