Move non-theme-specific inline editor styles into core.

Fix for #8571 (targeting release branch).

@larz0 can you take a look?

@mackenza does this do the trick for you?
Is the second selector here needed anymore?  The fix for #8541 made `.CodeMirror-linenumbers` opaque, and afaict that covers the entire bounds of `.CodeMirror-gutters`.  (And it it didn't, then part of #8541 would still repro, I think...)