Theme: implement and adopt service for handling ColorSchemes (dark, light, hc)

[bpasero]

The idea is to introduce central methods in IHostService to:

• figure out the current color scheme
• register to events when color schemes change

Idea for ColorScheme is an enum of:

• DEFAULT
• DARK
• HIGHT_CONTRAST

Implementation depends on the runtime:

• Electron: nativeTheme allows to figure out high contrast and dark scheme preference
• Browser: we can currently only detect preference for dark theme via a CSS check

[bpasero]

@aeschli please check #105716 for a first cut of support in the IHostService. Unfortunately I was not able to move the listener into IThemeService due to some cyclic dependencies, so maybe we have to put this into a separate workbench contribution that gets instantiated in the right lifecycle phase.

Things to do:

• move into theme component
• wire in the listener to dark theme changes in native environment
• figure out if we can support color scheme changes and HC detection logic somehow into the browser

[aeschli]

We now have a separate IHostColorSchemeService with a browser and electron variant.

The browser uses the experimental force-colors to detect hc: https://drafts.csswg.org/css-color-adjust-1/#forced-colors-mode
Forced colors is already enabled in Edge, and can be enabled in Chrome.
We also now set the rule forced-color-adjust: none so that we stay in control of all the colors.

[bpasero]

I suggest to either put this on test plan or verify.

[aeschli]

I'll put this on the test plan, there's enough to test