Add isolate node prop

FEATURE: The new `NodeProp.isolate` can be used by parser to signal
that a given node should be treated as isolated when it comes to bidirectional
text rendering.

src/tree.ts

@@ -78,6 +78,21 @@ export class NodeProp<T> {
   /// `"Expression"` group).
   static group = new NodeProp<readonly string[]>({deserialize: str => str.split(" ")})
 
+  /// Attached to nodes to indicate these should be
+  /// [displayed](https://codemirror.net/docs/ref/#language.syntaxTree)
+  /// in a bidirectional text isolate, so that direction-neutral
+  /// characters on their sides don't incorrectly get associated with
+  /// surrounding text. You'll generally want to set this for nodes
+  /// that contain arbitrary text, like strings and comments, and for
+  /// nodes that appear _inside_ arbitrary text, like HTML tags. When
+  /// not given a value, in a grammar declaration, defaults to
+  /// `"auto"`.
+  static isolate = new NodeProp<"rtl" | "ltr" | "auto">({deserialize: value => {
+    if (value && value != "rtl" && value != "ltr" && value != "auto")
+      throw new RangeError("Invalid value for isolate: " + value)
+    return (value as any) || "auto"
+  }})
+
   /// The hash of the [context](#lr.ContextTracker.constructor)
   /// that the node was parsed in, if any. Used to limit reuse of
   /// contextual nodes.