Symbols: Edit Symbol opens the symbol in its own editing mode, and every instance follows each edit

Squashed from #5064 onto main.

crates/doc/src/document.rs

@@ -47,6 +47,7 @@ impl Document {
             patterns,
             pattern_edit,
             mask_edit,
+            symbol_edit,
             next_id: _,
             unknown,
             page_isolate,
@@ -113,6 +114,7 @@ impl Document {
                 (a, b) => a.is_none() && b.is_none(),
             }
             && *mask_edit == other.mask_edit
+            && *symbol_edit == other.symbol_edit
             && *unknown == other.unknown
             && *page_isolate == other.page_isolate
             && *page_knockout == other.page_knockout
@@ -221,6 +223,7 @@ impl Document {
             patterns: vec![],
             pattern_edit: None,
             mask_edit: None,
+            symbol_edit: None,
             next_id: 1,
             unknown: Default::default(),
             page_isolate: false,