runtime(vim): Update base-syntax, match continued strings and tail comments

Continued strings are currently only matched after operators, in
parenthesised expressions and in function call argument lists.

closes: #14975

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim_comment.vim

@@ -31,6 +31,10 @@ autocmd BufNewFile * {
   \ continuing comment
   \ continuing comment
 
+let foo = 42 " commment
+  \ continuing comment
+  \ continuing comment
+
 " :Foo
       \ arg1
       "\ comment