Enable incremental webpack compilation by default

What does this Merge Request do and why?

This enables incremental webpack compilation by default.

It also adds a webpack.incremental_ttl configuration value which determines the number of days that page bundles are considered "recent", and should be eagerly compiled. This number represents the trade-off between lazy/eager compilation versus low/high memory consumption of the webpack development server. A higher number means fewer pages needing to be compiled on demand, at the cost of higher memory consumption. A lower number means lower memory consumption, at the cost of more pages being compiled on demand. A value of 0 means that all pages in your history, regardless of how long ago you visited them, are eagerly compiled.

Depends on gitlab!68956 (merged).

How to setup and validate locally (strongly suggested)

Follow the instructions in gitlab!68956 (merged), but instead of modifying the DEV_SERVER_INCREMENTAL_TTL environment variable via env.runit, set the new webpack.incremental_ttl configuration value in gdk.yml.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.