Add code insights lang stats creation UI

Created by: vovakulikov

Close https://github.com/sourcegraph/sourcegraph/issues/20481

The third of the three PRs for insights creation UI.

1. Creation UI for search based insight. https://github.com/sourcegraph/sourcegraph/pull/20032
2. Introduction page for creation UI. https://github.com/sourcegraph/sourcegraph/pull/20482
3. This Pull Request

This PR is about to add form for lang stats insights. You can find design layouts here

How to test

1. Open dev tools and set enableCodeInsightCreationUI field in local storage to true.
2. Go to /insight/create-intro and click create lang stats insight.