Prevent overriding of Disposable.dispose

A common mistake made was overriding dispose and not calling super.dispose,
so a TS trick is now used to move it to a readonly property instead of a
member method which prevents it being overridden.

src/common/parser/EscapeSequenceParser.test.ts

@@ -87,7 +87,7 @@ class TestEscapeSequenceParser extends EscapeSequenceParser {
     }
   }
   public mockOscParser(): void {
-    this._oscParser = oscPutParser;
+    (this as any)._oscParser = oscPutParser;
   }
   public identifier(id: IFunctionIdentifier): number {
     return this._identifier(id);