patch 9.2.0502: runtime(netrw): bookmark handling can be improved

patch 9.2.0502: runtime(netrw): bookmark handling can be improved

Problem:  To goto or delete a bookmark, one needs to prefix a count
          for the bookmark number (e.g., "2gb" to open bookmark#2).
          As the bookmark list gets or deletes entries, the numbers
          keep changing, requiring listing the bookmarks with qb to
          discover the desired bookmark number. Typing gb or mB
          without a count targets g:netrw_bookmarklist[-1].
Solution: If no count is given to gb or mB, list all bookmarks and
          prompt for a number using inputlist(), similar to tag jump
          with g].

closes: #20211

Signed-off-by: J. Paulo Seibt <jpseibt@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>