Collect source text from ParseCallback during parsing

[ALancki1]

This change collects source text returned by ParseCallback during parsing and stores it in the resulting Tree.

Previously, trees created from ParseCallback did not retain source text, causing methods like getText() and Node.getText() to return null. This change enables text-related APIs for callback-based parsing.

This also updates the callback parsing test to assert the new behavior.

Fixes #126

[ALancki1]

Thanks for taking a look. From my diff, the intended changes in Parser.java are only:

• collecting callback text into a StringBuilder
• appending each returned chunk
• passing the collected text into Tree

I don’t think I changed the null-language check itself. Were you pointing to a specific issue with the added StringBuilder change in that hunk?

[ObserverOfTime]

Using a byte buffer might be more efficient than a string builder.
In any case, this needs to be benchmarked before and after.

[ALancki1]

Good point. I switched this to collect bytes instead of using a StringBuilder, and I also used the callback offset to avoid duplicating overlapping chunks.