editor: Add SelectPageUp/SelectPageDown actions (#13272)

This adds two new actions to `editor`:

- `editor::SelectPageUp`
- `editor::SelectPageDown`

On Linux they're bound by default to `shift-pageup` and
`shift-pagedown`, which matches VS Code and JetBrains.

Release Notes:

- N/A

crates/editor/src/scroll.rs

@@ -330,6 +330,11 @@ impl Editor {
         self.scroll_manager.visible_line_count
     }
 
+    pub fn visible_row_count(&self) -> Option<u32> {
+        self.visible_line_count()
+            .map(|line_count| line_count as u32 - 1)
+    }
+
     pub(crate) fn set_visible_line_count(&mut self, lines: f32, cx: &mut ViewContext<Self>) {
         let opened_first_time = self.scroll_manager.visible_line_count.is_none();
         self.scroll_manager.visible_line_count = Some(lines);