Restore missing undo support

Sources/IBeam/MockTextSystem.swift

@@ -89,6 +89,6 @@ public final class MockTextSystem : TextSystem {
 	}
 
 	public func applyMutation(_ range: TextRange, string: AttributedString) -> MutationOutput<TextRange>? {
-		partialSystem.applyMutation(range, string: string)
+		partialSystem.applyMutation(in: range, string: string, undoManager: nil)
 	}
 }

Sources/IBeam/MutableStringPartialSystem.swift

@@ -82,10 +82,32 @@ extension MutableStringPartialSystem {
 		content.endEditing()
 	}
 
-	public func applyMutation(_ range: NSRange, string: AttributedString) -> MutationOutput<NSRange>? {
+//	public func applyMutation(_ range: NSRange, string: AttributedString) -> MutationOutput<NSRange>? {
+//		let nsAttrString = NSAttributedString(string)
+//		let length = nsAttrString.length
+//
+//		content.replaceCharacters(in: range, with: nsAttrString)
+//
+//		let delta = length - range.length
+//		let position = min(range.lowerBound + length, content.length)
+//
+//		let newSelection = NSRange(position..<position)
+//
+//		return MutationOutput<NSRange>(selection: newSelection, delta: delta)
+//	}
+
+	public func applyMutation(in range: NSRange, string: AttributedString, undoManager: UndoManager?) -> MutationOutput<NSRange> {
 		let nsAttrString = NSAttributedString(string)
 		let length = nsAttrString.length
 
+		let existingString = AttributedString(content.attributedSubstring(from: range))
+
+		undoManager?.registerMainActorUndo(withTarget: content, handler: { target in
+			let existingNSAttrString = NSAttributedString(existingString)
+
+			target.replaceCharacters(in: range, with: existingNSAttrString)
+		})
+
 		content.replaceCharacters(in: range, with: nsAttrString)
 
 		let delta = length - range.length