runtime(rst): Add support for rst_minlines

runtime(rst): Add support for rst_minlines

Following the approach used in other syntax definitions, add support for
defining the "syntax sync minlines=..." values for rst files in the
users' ~/.vimrc files, to allow the users to adjust that value in case
syntax highlighting stops working for some of the files they edit.

related: #18566

Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Marshall Ward <marshall.ward@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>