runtime(vim): Update base-syntax, match multiline continued comments (#13936)

Match multiline (continued) line comments.

Continued tail comments are not supported yet.

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

runtime/syntax/testdir/input/vim9_comment.vim

@@ -0,0 +1,64 @@
+vim9script
+
+# Vim comments
+
+# Vim9-script comment
+
+ # string only recognised with leading char
+ "useless string"
+
+function! Foo()
+  " Legacy-script comment
+  # 42 " comment
+endfunction
+
+def! Bar()
+  # Vim9-script comment
+  "useless string" # comment
+enddef
+
+command -count FooCommand {
+  # Vim9-script comment
+  "useless string" # comment
+}
+
+autocmd BufNewFile * {
+  # Vim9-script comment
+  "useless string" # comment
+}
+
+
+# Multiline comments
+
+# comment
+  \ continuing comment
+  \ continuing comment
+
+# :Foo
+      \ arg1
+      #\ comment
+      \ arg2
+
+echo "TOP"
+
+
+# Line-continuation comments
+
+:Foo
+      #\ line continuation comment
+      \ arg1
+      #\ line continuation comment
+      \ arg2
+
+
+# Issue: #13047
+
+if !exists(":DiffOrig")
+  command DiffOrig vert new | set bt=nofile | r ++edit %% | 0d_ | diffthis
+		  \ | wincmd p | diffthis
+endif
+
+
+# Issue: #11307 and #11560
+
+# This is what we call " blah