GN-6404: fix behaviour of embedded editor keymap

[elpoelma]

Overview

This PR solves issues with the keymap initialization of an embedded editor instance:

• The default keymap of an embedded editor is now correctly initialized
• An additional shift+Enter handler has been added to the default embedded editor keymap
• Only run leave-on-enter modifier when Enter has been pressed without modifier keys (Ctrl, Alt, Shift or Cmd)

connected issues and PRs:

GN-6404

How to test/reproduce

Before this fix

• Set the allowed content of a link to inline*
• Insert a link with some text inside
• Notice that none of the editor shortcuts work inside the link (Mod+b, Enter, Mod+i etc.)

After this fix

• Set the allowed content of a link to inline*
• Disable the leave-on-enter modifiers in the link component
• Insert a link with some text inside
• Notice that the editor shortcuts work inside the link (Mod+b, Enter, Mod+i etc.), as well as shift+Enter

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations

[abeforgit]

gonna merge this but some notes:

• adding newline at end of node looks like it does nothing the first time, but this is because browsers dont' render a trailing
  -> I don't think this is a regression
• I misunderstood the fix, my idea was to have leave-on-enter behavior for enter, and hard-break on shift-enter. But I thought this because I thought we added leave-on-enter for variable nodes as well as links, it seems we didn't.