Implement simplest linear check (which will work until joni opts some stack operations to be linear via caching)

Author: enebo

src/org/joni/Regex.java

@@ -507,4 +507,8 @@ public void setUserObject(Object object) {
     public Object getUserObject() {
         return userObject;
     }
+
+    public boolean isLinear() {
+        return !requireStack;
+    }
 }