Merge Trailing Line on Attachments, Select Attachments

[thecoolwinter]

Description

Fixes a bug with text attachments that ended on the newline character in a line. Eg:

A
B

Text attachment added for range (0..<2) should result in:

[Attachment]B

Right now it results in the following visible lines.

[Attachment]B

Also introduces the ability for text attachments to respond to being selected and draw their contents differently.

Related Issues

• ✨ Code Folding CodeEditSourceEditor#43

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots

With new behavior:

Screen.Recording.2025-06-03.at.10.15.54.AM.mov