multi_buffer: Split multi_buffer into more modules (#41033)

There are a of separate APIs in this, partially interleaved making it
difficult to grasp.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -7,6 +7,7 @@ use parking_lot::RwLock;
 use rand::prelude::*;
 use settings::SettingsStore;
 use std::env;
+use std::time::{Duration, Instant};
 use util::RandomCharIter;
 use util::rel_path::rel_path;
 use util::test::sample_text;
@@ -2984,7 +2985,7 @@ fn test_history(cx: &mut App) {
     });
     let multibuffer = cx.new(|_| MultiBuffer::new(Capability::ReadWrite));
     multibuffer.update(cx, |this, _| {
-        this.history.group_interval = group_interval;
+        this.set_group_interval(group_interval);
     });
     multibuffer.update(cx, |multibuffer, cx| {
         multibuffer.push_excerpts(