GN-4587: custom gap-cursor plugin

[elpoelma]

Overview

This PR introduces a custom gap-cursor plugin containing several fixes compared to the original version:

• The click handler has been replaced by a mousedown handler in order to intercept a click event earlier
• The types of the GapCursor class have been fixed
• Addition of a fix when resolving the position returned by view.posAtCoords.

connected issues and PRs:

Solves the gap-cursor part of GN-4587

How to test/reproduce

• Open-up the dummy app
• Insert a node with the isolating attribute (like a table) at the start of the document
• Notice that both in firefox and chromium-based browsers, it is now possible to click before/after the table in order to display a gap-cursor

Challenges/uncertainties

• This PR introduce an additional function resolvePosition which resolves the position returned by view.posAtCoords. Ideally, this function is not necessary, as it tries to circumvent the issue with wrong positions returned by view.posAtCoords.
• The fixes included in this PR should normally work in both firefox and chromium-based browsers.

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