terminal: Add scrollbar (#23256)

Closes #4798

This PR implements a scrollbar for the terminal by turning
`ScrollableHandle` into a trait, allowing us to implement a custom
scroll handle, `TerminalScrollHandle`. It works by converting terminal
lines into pixels that `ScrollableHandle` understands. When
`ScrollableHandle` provides a changed offset (e.g., when you drag the
scrollbar), we convert this pixel offset back into the number of lines
to scroll and update the terminal content accordingly.

While the current version works as expected, I believe the scrollbar's
offset updates could potentially be turned into an event. This event
could then be subscribed to in `TerminalView`, not needing to update the
terminal's offset in the `render` method as it might have performance
implications. Further ideas on this are welcome.

Preview:


https://github.com/user-attachments/assets/560f0aac-4544-4007-8f0b-8833386f608f

Todo:

- [x] Experiment with custom scrollbar responding to terminal mouse
scroll
- [x] Refactor existing scrollbar handle into a trait  
- [x] Update terminal to use the scrollbar trait instead of a custom
scrollbar implementation
- [x] Figure out how scrollbar events like mouse drag should notify the
terminal to update its state
- [x] Code clean up
- [x] Scrollbar hide setting for terminal

Release Notes:

- Added scrollbar to the terminal

crates/terminal/src/terminal_settings.rs

@@ -47,6 +47,40 @@ pub struct TerminalSettings {
     pub detect_venv: VenvSettings,
     pub max_scroll_history_lines: Option<usize>,
     pub toolbar: Toolbar,
+    pub scrollbar: ScrollbarSettings,
+}
+
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+pub struct ScrollbarSettings {
+    /// When to show the scrollbar in the terminal.
+    ///
+    /// Default: inherits editor scrollbar settings
+    pub show: Option<ShowScrollbar>,
+}
+
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+pub struct ScrollbarSettingsContent {
+    /// When to show the scrollbar in the terminal.
+    ///
+    /// Default: inherits editor scrollbar settings
+    pub show: Option<Option<ShowScrollbar>>,
+}
+
+/// When to show the scrollbar in the terminal.
+///
+/// Default: auto
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+#[serde(rename_all = "snake_case")]
+pub enum ShowScrollbar {
+    /// Show the scrollbar if there's important information or
+    /// follow the system's configured behavior.
+    Auto,
+    /// Match the system's configured behavior.
+    System,
+    /// Always show the scrollbar.
+    Always,
+    /// Never show the scrollbar.
+    Never,
 }
 
 #[derive(Clone, Debug, Default, Serialize, Deserialize, JsonSchema)]
@@ -187,6 +221,8 @@ pub struct TerminalSettingsContent {
     pub max_scroll_history_lines: Option<usize>,
     /// Toolbar related settings
     pub toolbar: Option<ToolbarContent>,
+    /// Scrollbar-related settings
+    pub scrollbar: Option<ScrollbarSettingsContent>,
 }
 
 impl settings::Settings for TerminalSettings {