`grep-next-match` works only on the `*grep*` buffer so it can't be used
on buffers that were preserved by renaming or on other grep-flavored
buffers created by 3rd party plugins kakoune-find and kakoune-lsp,
like `*find*` and `*references*`.
Let's generalize `grep-next-match` with a `jump-next` command that
takes a buffer argument.
This renames some options but I think they're not commonly used.
kakoune-lsp is an exception that uses grep_current_line but it's no big
deal, things will fail loud and early if options are missing.
Since grep.kak and friends now depend on jump.kak, move the jumpclient
declaration there as well.
