[java] Implement Join lines action for text block

Support ending \, insert \n otherwise (to preserve semantics)
IDEA-337178 Join lines: support terminating \ in text blocks

GitOrigin-RevId: 2acb64443b83a27f29a969360b2cbbe34f7ea7d4

java/java-tests/testData/codeInsight/joinLines/JoinTextBlockBackSlash2_after.java

@@ -0,0 +1,7 @@
+class A {
+  void test() {
+    String s = """
+      Hello  World!
+      """;
+  }
+}