Don't perform full syntax update on edit

[hbina]

Only perform a partial scan of the syntax and update the old one instead of replacing it entirely.
This will help a lot with editing large files.
There's an issue with this method however when dealing with syntax changes that are very big.
For example, commenting out a huge block of code will not trigger a syntax update for the block of code.
So there needs to be a better way to figure out which section of the code needs to be updated.

Alternative solutions:

1. Walk the syntax tree itself.
2. Perform the full update on a separate thread and send a command to update the syntax tree in the UI thread.

Similar optimization is also possible here https://github.com/lapce/lapce/blob/master/lapce-core/src/syntax/mod.rs#L280
The cursor will currently walk the entire syntax tree instead of only the relevant changes.

• Added an entry to CHANGELOG.md if this change could be valuable to users