generate-theme creates starter kit theme using VERSION (which is only for core)

Problem/Motivation

If you use php core/scripts/drupal generate-theme to generate a starter kit theme, you end up with a custom theme that uses VERSION for various version strings (both in the .info.yml and .library.yml files). This constant should only be used for core themes, which something generated with this script by definition is not. 😅

Steps to reproduce

1. Checkout 10.1.x core for local development.
2. Run php core/scripts/drupal generate-theme foo --path themes/custom
3. Inspect themes/custom/foo/foo.info.yml and themes/custom/foo/foo.libraries.yml.
4. Find a bunch of references to VERSION.

Proposed resolution

generate-theme should create everything using 1.0.0-alpha1 as the version string, not core's VERSION constant.

Remaining tasks

1. Do it.
2. Tests?
3. Reviews/refinements.
4. RTBC.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

TBD. Probably not?