Build: Use Statoscope webpack plugin for analysis

[jackw]

What is this feature?

Adds a stats webpack conf that uses statoscope webpack plugin to help analyse bundles / chunks...

Why do we need this feature?

To get a better understanding of what it loading when and why.

Who is this feature for?

Grafana developers

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:
Clone this branch and run yarn stats and wait for the plugin output to open in your browser.

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.