[kbn/optimizer] style performance improvements and expanded v8 theme support

[spalger]

The @kbn/optimizer is currently building light and dark versions of every .scss stylesheet in the Kibana repo every time the optimizer is run, regardless of whether developers every need the dark mode styles. This slows down development and would be much worse when we start building 8.0 versions of these styles too.

In order to expand support for more v8 theme development and to get some needed performance improvements in the @kbn/optimizer we are planning to limit the number of style variations the @kbn/optimizer will create by default, while enabling it to make more variations. We'll do this by introducing a KBN_STYLE_THEME environment variable that the @kbn/optimizer will read. It's default value will be v7light, it can be set to * or a comma separated list of v7light, v7dark, v8light, and v8dark.

When a developer switches to a theme that is not being built an error will be thrown in the browser instructing the user how to fix the issue, so it should be relatively easy to discover why things aren't working.

• @cchaos Add rule to SassLint that outlaws _styling_constants.scss imports
• @spalger [kbn/optimizer] only build specified themes #70389 Add KBN_OPTIMIZER_THEME config for @kbn/optimizer that only builds specific style types
• @cchaos Split up the styling constants into a unique file per style tag
• @spalger Import correct style constants based on style tag and remove sassOptions.importer

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)