runtime(java): Optionally highlight the :: token for method references

This token will be highlighted, similar to the arrow of
lambda expressions, whenever "g:java_highlight_functions" is
defined.

Also:

- Improve the recognition of _switch-case_ labels
  (D-Cysteine).
- Remove insignificant empty statements in syntax test
  files.

closes: #15322

References:
https://docs.oracle.com/javase/specs/jls/se21/html/jls-15.html#jls-15.13
https://github.com/fleiner/vim/pull/1

Co-authored-by: D-Cysteine <54219287+D-Cysteine@users.noreply.github.com>
Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/java_lambda_expressions_signature.java

@@ -136,7 +136,7 @@ class LambdaExpressions$Tests	// JDK 21+.
 			case String str_		-> str_;
 			}):			{ echo(str); break; }
 		case null: default:		{ echo("Other"); }
-		};
+		}
 
 		echo(switch (null) {
 			case String str when !"<empty>".equals(