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_enfoldment.java

@@ -39,7 +39,7 @@ class FoldingTests {
 				break;
 			}
 			default: ;
-		};
+		}
 	}
 
 	{ Object bb = ((Object) new byte[]{}); }