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/selection_command.rs

@@ -0,0 +1,98 @@
+use crate::assistant_panel::selections_creases;
+use anyhow::{anyhow, Result};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandContent, SlashCommandEvent,
+    SlashCommandOutputSection, SlashCommandResult,
+};
+use futures::StreamExt;
+use gpui::{AppContext, Task, WeakView};
+use language::{BufferSnapshot, CodeLabel, LspAdapterDelegate};
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+use ui::{IconName, SharedString, WindowContext};
+use workspace::Workspace;
+
+pub(crate) struct SelectionCommand;
+
+impl SlashCommand for SelectionCommand {
+    fn name(&self) -> String {
+        "selection".into()
+    }
+
+    fn label(&self, _cx: &AppContext) -> CodeLabel {
+        CodeLabel::plain(self.name(), None)
+    }
+
+    fn description(&self) -> String {
+        "Insert editor selection".into()
+    }
+
+    fn icon(&self) -> IconName {
+        IconName::Quote
+    }
+
+    fn menu_text(&self) -> String {
+        self.description()
+    }
+
+    fn requires_argument(&self) -> bool {
+        false
+    }
+
+    fn accepts_arguments(&self) -> bool {
+        true
+    }
+
+    fn complete_argument(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _cancel: Arc<AtomicBool>,
+        _workspace: Option<WeakView<Workspace>>,
+        _cx: &mut WindowContext,
+    ) -> Task<Result<Vec<ArgumentCompletion>>> {
+        Task::ready(Err(anyhow!("this command does not require argument")))
+    }
+
+    fn run(
+        self: Arc<Self>,
+        _arguments: &[String],
+        _context_slash_command_output_sections: &[SlashCommandOutputSection<language::Anchor>],
+        _context_buffer: BufferSnapshot,
+        workspace: WeakView<Workspace>,
+        _delegate: Option<Arc<dyn LspAdapterDelegate>>,
+        cx: &mut WindowContext,
+    ) -> Task<SlashCommandResult> {
+        let mut events = vec![];
+
+        let Some(creases) = workspace
+            .update(cx, selections_creases)
+            .unwrap_or_else(|e| {
+                events.push(Err(e));
+                None
+            })
+        else {
+            return Task::ready(Err(anyhow!("no active selection")));
+        };
+
+        for (text, title) in creases {
+            events.push(Ok(SlashCommandEvent::StartSection {
+                icon: IconName::TextSnippet,
+                label: SharedString::from(title),
+                metadata: None,
+            }));
+            events.push(Ok(SlashCommandEvent::Content(SlashCommandContent::Text {
+                text,
+                run_commands_in_text: false,
+            })));
+            events.push(Ok(SlashCommandEvent::EndSection { metadata: None }));
+            events.push(Ok(SlashCommandEvent::Content(SlashCommandContent::Text {
+                text: "\n".to_string(),
+                run_commands_in_text: false,
+            })));
+        }
+
+        let result = futures::stream::iter(events).boxed();
+
+        Task::ready(Ok(result))
+    }
+}