runtime(lf): use syn iskeyword in syntax script

runtime(lf): use syn iskeyword in syntax script

Sets 'syn iskeyword' in syntax/lf.vim to fix the missing lf keyword
highlighting in lines like 'map e :open; open' (first 'open' not
highlighted).

applies PR andis-sprinkis/lf-vim#21 by @joelim-work
closes: andis-sprinkis/lf-vim#14

Co-authored-by: Joe Lim <50560759+joelim-work@users.noreply.github.com>
Signed-off-by: Andis Spriņķis <andis@sprinkis.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>