Replace Default trait bound with a zero function on Summary/Dimension (#17975)

This lets us provide a context when constructing the zero value. We need
it so we can require anchors to be associated with a buffer id, which
we're doing as part of simplifying the multibuffer API.

Release Notes:

- N/A

Co-authored-by: Nathan <nathan@zed.dev>

crates/text/src/operation_queue.rs

@@ -34,7 +34,7 @@ impl<T: Operation> Default for OperationQueue<T> {
 
 impl<T: Operation> OperationQueue<T> {
     pub fn new() -> Self {
-        OperationQueue(SumTree::new())
+        OperationQueue(SumTree::default())
     }
 
     pub fn len(&self) -> usize {
@@ -58,7 +58,7 @@ impl<T: Operation> OperationQueue<T> {
 
     pub fn drain(&mut self) -> Self {
         let clone = self.clone();
-        self.0 = SumTree::new();
+        self.0 = SumTree::default();
         clone
     }
 
@@ -70,6 +70,10 @@ impl<T: Operation> OperationQueue<T> {
 impl Summary for OperationSummary {
     type Context = ();
 
+    fn zero(_cx: &()) -> Self {
+        Default::default()
+    }
+
     fn add_summary(&mut self, other: &Self, _: &()) {
         assert!(self.key < other.key);
         self.key = other.key;
@@ -90,6 +94,10 @@ impl<'a> Add<&'a Self> for OperationSummary {
 }
 
 impl<'a> Dimension<'a, OperationSummary> for OperationKey {
+    fn zero(_cx: &()) -> Self {
+        Default::default()
+    }
+
     fn add_summary(&mut self, summary: &OperationSummary, _: &()) {
         assert!(*self <= summary.key);
         *self = summary.key;