common: Add #contains

stringhold-common/src/main/java/com/kohlschutter/stringhold/StringHolder.java

@@ -742,8 +742,18 @@ default int indexOf(int c) {
   }
 
   /**
-   * Returns the index within this StringHolder of the first occurrence of the specified
-   * CharSequence, or {@code -1} if not found.
+   * Checks if this {@link StringHolder} contains the given {@link CharSequence}.
+   * 
+   * @param s The char sequence to look for.
+   * @return {@code true} if found (also if the sequence is empty).
+   */
+  default boolean contains(CharSequence s) {
+    return indexOf(s.toString()) >= 0;
+  }
+
+  /**
+   * Returns the index within this {@link StringHolder} of the first occurrence of the specified
+   * {@link CharSequence}, or {@code -1} if not found.
    *
    * @param str The char sequence to look for.
    * @return The position, or {@code -1} if not found.

stringhold-common/src/test/java/com/kohlschutter/stringhold/StringHolderTest.java

@@ -1423,4 +1423,10 @@ public void testIndexOfSelf() throws Exception {
     sh = StringHolder.withSupplier(() -> "");
     assertEquals(0, sh.indexOf(sh));
   }
+
+  @Test
+  public void testContains() throws Exception {
+    assertTrue(StringHolder.withContent("Foo bar").contains("bar"));
+    assertFalse(StringHolder.withContent("Foo bar").contains("baz"));
+  }
 }