Incorrect and slow parsing of base64 encoded CSS property values

[marlass]

When running generateJSON from @tiptap/html we noticed huge performance problem on docs containing with inline styles on elements containing base64 encoded images.

The problem first pointed to prosemirror-model, but after change there the problem moved to zeed-dom. By looking at the code it seems to be almost identical (incorrect regex parsing of styles declaration). Prosemirror issue

Changes introduces in https://github.com/ProseMirror/prosemirror-model/commit/899a98e62db1b2262bb4c67e4a401bc39b530385 exposed also other problems in style getter. It should expose object with CSSStyleDeclaration interface, but it's plain object which is now incompatible with prosemirror-model.

Due to dependencies between Tiptap, prosemirror and zeed-dom fixing this seems quite important for future releases compatibility.