@grafana/toolkit: cleanup (#21441)

* Clarify readme for extended webpack config

* Add support for ES2019 features

* Build task: path.resolve → resolvePath

* Build task: avoid fs race conditions

* Build task: use async fs functions

* Build task: use rimraf directly

... because depending on the workspace parent to have it installed is fragile, and a child process is always slower.

* Build task: misc

tsconfig.json

@@ -3,7 +3,7 @@
     "moduleResolution": "node",
     "outDir": "public/dist",
     "target": "es5",
-    "lib": ["es6", "dom"],
+    "lib": ["es2019", "dom"],
     "rootDirs": ["public/"],
     "jsx": "react",
     "module": "esnext",