Implement ToCss for PropertyDeclaration, cleanup CSSStyleDeclaration serialization

[brunoabinader]

Sub-task for #4440.

[hoppipolla-critic-bot]

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[brunoabinader]

r? @SimonSapin

[SimonSapin]

Sorry I was unclear. The idea behind #4455 is to stop using Show for serialization entirely. (And use ToCss instead.) That is, PropertyDeclaration::value and DeclaredValue::specified_value should be replaced with ToCss implementations, and should recursively call .to_css(destination) on their components instead of using format!.