[ChromeVox] Convert EditableLine to TypeScript

AX-Relnotes: n/a.
Bug: b/267329383
Change-Id: I784b8ad5acd5d528ead2bee3435bbcc60dc838da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5607488


Reviewed-by: Jacob Francis <francisjp@google.com>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/main@{#1313069}