Stop collecting declarations as soon as we detect a nested rule. Any declaration that come after the nested rules are now put in a separate CSSNestedDeclaration rule. We still need to handle the nesting logic for commented declarations, as highlighted by a test case in test_parseDeclarations.js. Other test cases there for nested rules are updated, and we add a CSSNestedDeclaration in a Rules view mochitest to make sure we don't get the declaration in the "top level" rule anymore. Differential Revision: https://phabricator.services.mozilla.com/D237520