Rework selection replacement

[dlqqq]

Description

• Removes "Replace selection" checkbox that shows underneath the chat input when the user selects a range of text
• Re-implements selection replacement by allowing users to both replace a text selection or an active cell, with the text selection taking priority.
• Implements a hamburger menu that shows next to each agent message, which allows replacing/inserting the entire response (not just a single code block) into the user's main panel.
• Fixes Remove "Replace selection" checkbox #865.

Demo

Screen.Recording.2024-07-12.at.1.58.26.PM.mov

[srdas]

@dlq It all looks good. I get either a selection replaced or the entire text replaced.

Nit: The replacement does not always take care of the highlighted text but all the lines. See the video below. I highlighted two lines of code by taking my cursor to the start of the third line, and it then replaces the third line also, even though it is not highlighted. See this video:
https://github.com/user-attachments/assets/cc66635b-cad3-44df-a5f4-59fe414006dd
If you highlight a part of the third line in the video, it will replace that part and not the whole line, and the same is true if you go across lines and highlight only parts of them.

[srdas]

Small nit -- all else looks good. If this is intended, please override the comment.