runtime(java): Recognise the {@snippet} documentation tag (#14271)

runtime(java): Recognise the {@snippet} documentation tag (#14271)

Remember that ‘code fragments are typically Java source
code, but they may also be fragments of properties files,
source code in other languages, or plain text.’  Therefore,
with these changes, markup tags are highlighted in the Java
source files (as external snippets) and in the {@snippet}
tags.

Also:

- Improve matching of the multi-line {@code} documentation
  tag with any contained balanced braces.
- Recognise the {@literal} documentation tag.
- Highlight stray blanks in comments.

Related to an enhancement proposal for PCRE-like callouts
discussed at https://github.com/vim/vim/issues/11217.

References:
https://openjdk.org/jeps/413
https://docs.oracle.com/en/java/javase/21/docs/specs/javadoc/doc-comment-spec.html

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>