editor: Improve outdent performance (#2800)

## Description

Outdent without a selection would copy the rest of the document just to
see if the string starts with `tab_indent`. Replaced with a cheap
iterator check instead.

## How to Test

Outdent in the editor example.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [x] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)