terminal: Expose selection in context and add keep_selection_on_copy setting (#33491)

Closes #21262

Introduces a new setting `keep_selection_on_copy`, which controls
whether the current text selection is preserved after copying in the
terminal. The default behavior remains the same (`true`), but setting it
to `false` will clear the selection after the copy operation, matching
VSCode's behavior.

Additionally, the terminal context now exposes a `selection` flag
whenever text is selected.

This allows users to match VSCode and other terminal's smart copy
behavior.

Release Notes:

- Expose `selection` to terminal context when there is text selected in
the terminal
- Add `keep_selection_on_copy` terminal setting. Can be set to false to
clear the text selection when copying text.

**VSCode Behavior Example:**

**settings.json:**
```json
  "terminal": {
    "keep_selection_on_copy": false
  },
```
**keymap.json:**
```json
  {
    "context": "Terminal && selection",
    "bindings": {
      "ctrl-c": "terminal::Copy"
    }
  }
```

crates/terminal/src/terminal.rs

@@ -898,7 +898,13 @@ impl Terminal {
 
             InternalEvent::Copy => {
                 if let Some(txt) = term.selection_to_string() {
-                    cx.write_to_clipboard(ClipboardItem::new_string(txt))
+                    cx.write_to_clipboard(ClipboardItem::new_string(txt));
+
+                    let settings = TerminalSettings::get_global(cx);
+
+                    if !settings.keep_selection_on_copy {
+                        self.events.push_back(InternalEvent::SetSelection(None));
+                    }
                 }
             }
             InternalEvent::ScrollToAlacPoint(point) => {

crates/terminal/src/terminal_settings.rs

@@ -40,6 +40,7 @@ pub struct TerminalSettings {
     pub alternate_scroll: AlternateScroll,
     pub option_as_meta: bool,
     pub copy_on_select: bool,
+    pub keep_selection_on_copy: bool,
     pub button: bool,
     pub dock: TerminalDockPosition,
     pub default_width: Pixels,
@@ -193,6 +194,10 @@ pub struct TerminalSettingsContent {
     ///
     /// Default: false
     pub copy_on_select: Option<bool>,
+    /// Whether to keep the text selection after copying it to the clipboard.
+    ///
+    /// Default: false
+    pub keep_selection_on_copy: Option<bool>,
     /// Whether to show the terminal button in the status bar.
     ///
     /// Default: true