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"
    }
  }
```

assets/settings/default.json

@@ -1292,6 +1292,8 @@
     // Whether or not selecting text in the terminal will automatically
     // copy to the system clipboard.
     "copy_on_select": false,
+    // Whether to keep the text selection after copying it to the clipboard
+    "keep_selection_on_copy": false,
     // Whether to show the terminal button in the status bar
     "button": true,
     // Any key-value pairs added to this list will be added to the terminal's