Bump `text-buffer` to v15

[DeeDeeG]

See:

• https://github.com/pulsar-edit/text-buffer/releases/tag/v15.0.0
• Promote “deleted” into its own state text-buffer#12 --> Enhance deleted state text-buffer#13
• Remove orphaned dependencies text-buffer#14

Could be nice to get these improvements here in core repo.

[savetheclocktower]

The text-buffer bump is happening as part of #1588. Simply bumping text-buffer will cause behavior changes around deleted files, so I'm intentionally bundling the bump with other code that introduces a config setting to bring the old behavior back.