GROOVY-6360: Add more String methods to avoid reflection

Just add `String` methods introduced by Java 11

src/main/java/org/codehaus/groovy/runtime/GStringImpl.java

@@ -28,6 +28,7 @@
 import java.lang.reflect.Method;
 import java.nio.charset.Charset;
 import java.util.Locale;
+import java.util.stream.Stream;
 
 /**
  * Default implementation of a GString used by the compiler. A GString consists
@@ -151,6 +152,30 @@ public boolean isEmpty() {
         return toString().isEmpty();
     }
 
+    public Stream<String> lines() {
+        return toString().lines();
+    }
+
+    public boolean isBlank() {
+        return toString().isBlank();
+    }
+
+    public String repeat(int count) {
+        return toString().repeat(count);
+    }
+
+    public String stripLeading() {
+        return toString().stripLeading();
+    }
+
+    public String stripTrailing() {
+        return toString().stripTrailing();
+    }
+
+    public String strip() {
+        return toString().strip();
+    }
+
     public int codePointAt(int index) {
         return toString().codePointAt(index);
     }

src/test/groovy/GStringTest.groovy

@@ -597,6 +597,17 @@ class GStringTest extends GroovyTestCase {
         assert gstring.toString() == 'Green eggs and ham'
     }
 
+    // GROOVY-6360
+    void testGStringDelegatingMethods() {
+        def gstring = " Hello, ${'world'}! "
+        assert gstring.lines().toList() == [' Hello, world! ']
+        assert !gstring.isBlank()
+        assert gstring.repeat(2) == " Hello, world!  Hello, world! "
+        assert gstring.stripLeading() == "Hello, world! "
+        assert gstring.stripTrailing() == " Hello, world!"
+        assert gstring.strip() == "Hello, world!"
+    }
+
     // GROOVY-7494
     void testGStringCoercionForArrayPutAt() {
         String[] fubar = new String[1]