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"
    }
  }
```

docs/src/configuring-zed.md

@@ -2564,6 +2564,7 @@ List of `integer` column numbers
     "alternate_scroll": "off",
     "blinking": "terminal_controlled",
     "copy_on_select": false,
+    "keep_selection_on_copy": false,
     "dock": "bottom",
     "default_width": 640,
     "default_height": 320,
@@ -2688,6 +2689,26 @@ List of `integer` column numbers
 }
 ```
 
+### Terminal: Keep Selection On Copy
+
+- Description: Whether or not to keep the selection in the terminal after copying text.
+- Setting: `keep_selection_on_copy`
+- Default: `false`
+
+**Options**
+
+`boolean` values
+
+**Example**
+
+```json
+{
+  "terminal": {
+    "keep_selection_on_copy": true
+  }
+}
+```
+
 ### Terminal: Env
 
 - Description: Any key-value pairs added to this object will be added to the terminal's environment. Keys must be unique, use `:` to separate multiple values in a single variable