Build: Optional skipping of typescript checking in dev bundler (#18772)

* Build: Removed typescript checking from dev bundler

- when switching branches and trying out features, TS checking and bundling are eating up a lot
of CPU
- most of us are using an editor with typechecking support
- this commit removes type checking from the dev bundler
- we still have type checking enabled as a precommit hook and as part of
the prod bundler

* Adds new npm script: start:noTsCheck

package.json

@@ -146,6 +146,7 @@
     "start": "grafana-toolkit core:start --watchTheme",
     "start:hot": "grafana-toolkit core:start --hot --watchTheme",
     "start:ignoreTheme": "grafana-toolkit core:start --hot",
+    "start:noTsCheck": "grafana-toolkit core:start --noTsCheck",
     "watch": "yarn start -d watch,start core:start --watchTheme ",
     "build": "grunt build",
     "test": "grunt test",