Function apply_content_change 

pub fn apply_content_change(
    text: &mut String,
    range: Option<Range>,
    new_text: &str,
)

Apply one LSP content change to a document’s text.

A None range is a whole-document replacement; a Some range is an incremental edit whose positions are in UTF-16 code units, converted to byte offsets through a fresh LineIndex over the current text.