refactor(themes): add dark variables, move base out of themes

[damyanpetev]

Refactored grid themes to components in igniteui-webcomponents with base loaded permanently instead of bundled with each theme.
Dark theme host variables copied from fromigniteui-webcomponents's third-party styles like:
https://github.com/IgniteUI/igniteui-webcomponents/blob/238c17763ab4c094cb0bb4701ee9ebd147c8b386/src/styles/themes/third-party/grid/bootstrap.dark.scss#L5-L14

Since those are added on top of the single light available before, kept the rest of the variables from that in each theme - those could be refactored into a common (as noted) but even like this the themes are smaller now without base and fully self-contained.

[coveralls]

Pull Request Test Coverage Report for Build 19357308278

Details

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.006%) to 96.479%

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Totals
Change from base Build 19301235053:	-0.006%
Covered Lines:	2622
Relevant Lines:	2706

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💛 - Coveralls

[simeonoff]

@damyanpetev I will approve this implementation for now, however, I'd like to move to consuming the colors from the grid schemas in the theming package. We need to create an issue for that, but I don't have the rights to do it myself.