Use rfind rather filter.next_back.

[WindSoilder]

I noticed CI error when running clippy:

warning: called `filter(..).next_back()` on an `DoubleEndedIterator`. This is more succinctly expressed by calling `.rfind(..)` instead
   --> src/core_editor/line_buffer.rs:273:9
    |
273 | /         self.lines[..self.insertion_point]
274 | |             .split_word_bound_indices()
275 | |             .filter(|(_, word)| !is_whitespace_str(word))
276 | |             .next_back()
    | |________________________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#filter_next
    = note: `#[warn(clippy::filter_next)]` on by default
help: try
    |
273 ~         self.lines[..self.insertion_point]
274 +             .split_word_bound_indices().rfind(|(_, word)| !is_whitespace_str(word))
    |

This pr is going to fix it