[feature] 'diff_changed_to' to notify only about new changes without including older stuff

**Version and OS**
0.49.3 running in docker

**Is your feature request related to a problem? Please describe.**
Telegram has a symbol limit per message. When you try sending long changes - they trim it.
Currently, if you use any of {{diff}}, {{diff_added}}, {{diff_removed}}, {{diff_full}} - it will show & notify about changes with following format: changed FROM (removed stuff here)
TO (new added stuff).
Problem is, i'm not always needed to see which change is changed 'from'.
Mikrotik posts changelogs of their RouterOS with removing and adding new stuff. If this change is too big - telegram will trim all the new added stuff and also partially old stuff.



**Describe the solution you'd like**
Adding 'diff_changed_to' would be nice.

**Describe the use-case and give concrete real-world examples**
Example of the url: https://changedetection.io/share/1yFcNngEl0Qa

TL;DR part: only notify about 'green' stuff (new), not all the 'red' (old) + 'green' (new)

Start by tracing how the existing {{diff}}, {{diff_added}}, {{diff_removed}}, and {{diff_full}} variables are generated and formatted for Telegram notifications. Add the requested {{diff_changed_to}} behavior so notifications contain only the new content, then verify that existing variables remain unchanged and that long Telegram messages preserve the new portion.

Written by the indexing model from the issue text.