editor: Add scroll_beyond_last_line setting (#11155)

Add `scroll_beyond_last_line` setting with 3 options: 

- `one_page`: The default (current) behaviour of scrolling one more page
beyond the last line.
<img width="568" alt="SCR-20240429-sxry"
src="https://github.com/zed-industries/zed/assets/126383/1effbee9-759f-4858-9022-83bbb208ef82">

- `off`: No scrolling beyond the last line. 
<img width="568" alt="SCR-20240429-syhv"
src="https://github.com/zed-industries/zed/assets/126383/5391b1d7-918d-43f3-8a6f-7642ef32d174">

- `vertical_scroll_margin`: Scroll beyond the last line by the same
number of lines as `vertical_scroll_margin`. Matches the behaviour of
keyboard scrolling.
<img width="568" alt="SCR-20240429-sypc"
src="https://github.com/zed-industries/zed/assets/126383/bb9cc928-e515-4503-88f7-e434c45d742f">

Release Notes:

- Added `scroll_beyond_last_line` setting
([#4962](https://github.com/zed-industries/zed/issues/4962)).

crates/editor/src/editor_settings.rs

@@ -15,6 +15,7 @@ pub struct EditorSettings {
     pub toolbar: Toolbar,
     pub scrollbar: Scrollbar,
     pub gutter: Gutter,
+    pub scroll_beyond_last_line: ScrollBeyondLastLine,
     pub vertical_scroll_margin: f32,
     pub scroll_sensitivity: f32,
     pub relative_line_numbers: bool,
@@ -116,6 +117,22 @@ pub enum MultiCursorModifier {
     CmdOrCtrl,
 }
 
+/// Whether the editor will scroll beyond the last line.
+///
+/// Default: one_page
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum ScrollBeyondLastLine {
+    /// The editor will not scroll beyond the last line.
+    Off,
+
+    /// The editor will scroll beyond the last line by one page.
+    OnePage,
+
+    /// The editor will scroll beyond the last line by the same number of lines as vertical_scroll_margin.
+    VerticalScrollMargin,
+}
+
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
 pub struct EditorSettingsContent {
     /// Whether the cursor blinks in the editor.
@@ -158,6 +175,10 @@ pub struct EditorSettingsContent {
     pub scrollbar: Option<ScrollbarContent>,
     /// Gutter related settings
     pub gutter: Option<GutterContent>,
+    /// Whether the editor will scroll beyond the last line.
+    ///
+    /// Default: one_page
+    pub scroll_beyond_last_line: Option<ScrollBeyondLastLine>,
     /// The number of lines to keep above/below the cursor when auto-scrolling.
     ///
     /// Default: 3.