chore(luarc): include VIMRUNTIME in workspace library

This commit adds the `$VIMRUNTIME` environment variable to
`workspace.library` in `.luarc.json`. The Lua language server does not
merge `workspace.library` in the configuration it receives from multiple
sources. As a result, only the `library` from `.luarc.json` gets used.

.luarc.json

@@ -10,6 +10,7 @@
   "workspace": {
     "checkThirdParty": "Disable",
     "library": [
+      "$VIMRUNTIME",
       "test/plenary"
     ],
     "ignoreDir": [