feat: only update CSS Vars if necessary

[vladitasev]

Whenever styles/links/adopted stylesheets are created for the CSS Variables, save info about:

• which runtime created the resource
• which theme the CSS vars are for (for theme-related variables, as there are also system CSS vars and such, which are theme independent)

Whenever styles/links/adopted stylesheets are about to be overwritten with new CSS Variables:

• always overwrite, if the theme was changed (as CSS Vars for the new theme must replace CSS Vars for the old theme for sure)
• but if the theme has not changed, only overwrite CSS Vars with ones from a newer version of the framework

For this new functionality to work, it is only necessary to pass the theme when calling createOrUpdateStyle

related to: https://github.com/SAP/ui5-webcomponents/issues/3781

[vladitasev]

This change can be merged, but the team is investigating other solutions (CSS-based/scoped custom registries), therefore closing for the time being.