runtime(vim): Update base-syntax, configurable comment string highlighting (#14931)

Allow highlighting of strings within comments to be disabled by setting
g:vimsyn_comment_strings to false.

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

runtime/syntax/testdir/input/vim9_ex_comment_strings.vim

@@ -0,0 +1,22 @@
+vim9script
+
+# Vim comment strings
+# VIM_TEST_SETUP let g:vimsyn_comment_strings = v:true
+
+# pre "string" post
+
+function Foo()
+  " pre "string" post
+endfunction
+
+def Bar()
+  # pre "string" post
+enddef
+
+command Foo {
+  # pre "string" post
+}
+
+autocmd BufNewFile * {
+  # pre "string" post
+}