runtime(vim): Update syntax and ftplugin files (#13924)

Improve matching of line-continuations and interspersed comments.

These are now also matched in multiline syntax command patterns,
dictionary literals, and parenthesised expressions and argument lists.

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

runtime/syntax/testdir/input/vim_line_continuation.vim

@@ -0,0 +1,49 @@
+" Vim line continuations with interspersed comments
+
+function Foo(
+      "\ param a
+      \ a,
+      "\ param b
+      \ b,
+      "\ param c
+      \ c
+      \)
+  echomsg
+	"\ start string
+	\ $"
+	"\ print a
+	\ a = {a:a},
+	"\ print b
+	\ b = {a:b},
+	"\ print c
+	\ c = {a:c}
+	"\ end string
+	\"
+endfunction
+
+call Foo(
+      "\ arg 1
+      \ 11,
+      "\ arg 2
+      \ 22,
+      "\ arg 3
+      \ 33
+      \)
+
+let dict = #{
+      "\ pair 1
+      \ a: 1,
+      "\ pair 2
+      \ b: 2,
+      "\ pair 3
+      \ c: 3
+      \}
+
+let array = [
+      "\ element 1
+      \ 1,
+      "\ element 2
+      \ 2,
+      "\ element 3
+      \ 3
+      \]