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>

docs/src/assistant/commands.md

@@ -13,6 +13,7 @@ Slash commands enhance the assistant's capabilities. Begin by typing a `/` at th
 - `/symbols`: Inserts the current tab's active symbols into the context
 - `/tab`: Inserts the content of the active tab or all open tabs into the context
 - `/terminal`: Inserts a select number of lines of output from the terminal
+- `/selection`: Inserts the selected text into the context
 
 ### Other Commands:
 
@@ -95,6 +96,14 @@ Usage: `/terminal [<number>]`
 
 - `<number>`: Optional parameter to specify the number of lines to insert (default is a 50).
 
+## `/selection`
+
+The `/selection` command inserts the selected text in the editor into the context. This is useful for referencing specific parts of your code.
+
+This is equivalent to the `assistant: quote selection` command ({#kb assistant::QuoteSelection}). See [Interacting with the Assistant](./assistant-panel.md#interacting-with-the-assistant)).
+
+Usage: `/selection`
+
 ## `/workflow`
 
 The `/workflow` command inserts a prompt that opts into the edit workflow. This sets up the context for the assistant to suggest edits to your code.