Add color token for error background

[miguelsolorio]

Fixes #16204 and adds a new color token for error background colors, editorError.background:

Followed the same naming convention as:

• editorError.border
• editorError.foreground

[aeschli]

The color needs to be transparent to work with other highlight colors. We should add that information to the description. There's also a boolean parameter on registerColor to signal that.
Example:

vscode/src/vs/editor/common/view/editorColorRegistry.ts

Line 16 in 07f0bad

export const editorRangeHighlight = registerColor('editor.rangeHighlightBackground', { dark: '#ffffff0b', light: '#fdff0033', hc: null }, nls.localize('rangeHighlight', 'Background color of highlighted ranges, like by quick open and find features. The color must not be opaque so as not to hide underlying decorations.'), true);

What about the other problem types? Warning, info, ...

[miguelsolorio]

@aeschli thanks for the pointer, I'll update this and also add the other types.

[miguelsolorio]

Updated and added editorWarning.background and editorInfo.background: