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/dumps/java_unfoldment_02.dump

@@ -8,7 +8,7 @@
 @16|b+0#af5f00255&|r|e|a|k|;+0#0000000&| @52
 @12|}| @61
 @12|d+0#af5f00255&|e|f|a|u|l|t|:+0#0000000&| |;| @52
-@8|}|;| @64
+@8|}| @65
 @4|}| @69
 @75
 @4|{| |O|b|j|e|c|t| |b@1| |=| |(@1|O|b|j|e|c|t|)| |n+0#af5f00255&|e|w| +0#0000000&|b+0#00e0003&|y|t|e|[+0#0000000&|]|{+0#ffffff16#ff404010|}|)+0#0000000#ffffff0|;| |}| @30