LibWeb: Implement and use BackgroundStyleValue

This one represents one secton of a `background` property, since it can
have multiple background values separated by commas. Eventually, we will
represent that as a List of BackgroundStyleValues.

Also modified some background-foo properties in StyleResolver so that
the is_background_x() functions could be removed.

I realized that our handling of var() in shorthand properties is wrong,
so have been removing the is_builtin_or_dynamic() calls from the parsing
code for shorthands. This broke our var() test page, so I have replaced
the use of 'background' with 'background-color' there.

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