runtime(java): Provide support for syntax preview features

runtime(java): Provide support for syntax preview features

Introduce a new API variable "g:java_syntax_previews" whose
value must be a list of syntax preview feature numbers.

Enumerate the currently supported numbers in a table at the
end of the documentation entry for "ft-java-syntax".

Also, disable the recognition of String Templates.  Despite
the withdrawal of this preview feature in its proposed form
from the upcoming JDK 23 release and the fact that the JDK
22 release is coming to EOL this September, an earlier
iteration of this preview feature was included in JDK 21
(LTS) whose EOL is projected to fall due in late 2028 and,
therefore, retain the current implementation.

Define "g:java_syntax_previews" and include number 430 in
its list to enable the recognition of String Templates:
------------------------------------------------------------
	let g:java_syntax_previews = [430]
------------------------------------------------------------

References:
https://openjdk.org/jeps/430 (Preview)
https://openjdk.org/jeps/459 (Second Preview)
https://openjdk.org/jeps/465 (Third Preview)
https://mail.openjdk.org/pipermail/amber-spec-experts/2024-April/004106.html

closes: #15579

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