Text completion callback for Copilot-style completions to support LLMs on co-writing.

**Is your feature request related to a problem? Please describe.**
In our LLMs era, every editor should have the ability to complete texts. This significantly improves writing speed and allows to get rid of typical grammar errors

**Describe the solution you'd like**
I would suggest to pass an optional callback to settings which is called every time the editor ready to accept completion. The callback should be triggered when the cursor moves (e.g. when user is typing new content or just changes position), and callback should accept text from zero position up to a cursor. Ideally have some debounce though this could be added on user-level.

**Describe alternatives you've considered**
I have implemented similar component using Quill, but I would like to have it for EasyMDE.
https://github.com/devforth/vue-suggestion-input?tab=readme-ov-file

**Additional context**
here is a sample how it should work:

The issue names no files or tests. Start by reviewing EasyMDE’s public settings API and cursor-change handling, then compare the linked vue-suggestion-input example. Done means an optional completion callback receives text from the document start to the cursor whenever the cursor moves, with debounce behavior explicitly decided.

Written by the indexing model from the issue text.