Refactor constructor and replaceSync() steps so CSS modules can reference them

[dandclark]

The steps to create a CSS module (spec draft) should reference the steps in this spec to create a constructable stylesheet and run replaceText(). This will help ensure that the behavior of CSS modules and constructable stylesheets remains consistent as much as possible.

To make this possible, this change refactors the implementations of these algorithms into separate sets of steps that CSS modules can reference directly. By referencing these new algorithms instead of the public methods, this makes it clear that the behavior of CSS modules shouldn't change if the public methods for CSSStyleSheet are replaced by script.

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