assistant: Implement /selection slash command (#19988)

- Closes #18868

## Summary
This PR introduces a new slash command `/selection` to enhance the
usability of the assistant's quote selection feature.

## Changes Made
1. Extracted a function from the `assistant: quote selection` action to
find the selected text and format it as an assistant section.
2. Created a new slash command `/selection` that utilizes the extracted
function to achieve the same effect as the existing `assistant: quote
selection` action.
3. Updated the documentation to include information about the new
`/selection` slash command.


Release Notes:

- Moved the text selection action to a slash command (`/selection`) in
the assistant panel

---------

Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>

crates/assistant/src/slash_command.rs

@@ -31,6 +31,7 @@ pub mod now_command;
 pub mod project_command;
 pub mod prompt_command;
 pub mod search_command;
+pub mod selection_command;
 pub mod streaming_example_command;
 pub mod symbols_command;
 pub mod tab_command;