runtime(vim): Update base-syntax, improve :menu{,translate} highlighting (#14162)

Improve :menu and :menutranslate highlighting.

- Match args to :menutranslation and :popup.
- Only highlight special notation in {rhs} of :menu, like :map.
- Allow line continuations in {rhs} of :menu and between {english} and
  {mylang} of :menutranslation, matching common usage.
- Bug fixes.

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

runtime/syntax/testdir/input/vim_ex_menutranslate.vim

@@ -0,0 +1,51 @@
+" Vim :menutranslate command
+
+menutranslate clear
+menutranslate clear | echo "Foo"
+menutranslate clear " comment
+
+menutranslate &Foo\ bar &FuBar | echo "Foo"
+
+menutranslate &Foo\ bar &FuBar " comment
+menutranslate \"&Foo"\ bar \"&FuBar
+menutranslate &Foo\ "bar" &FuBar
+
+menutranslate &Foo\ bar
+      \ &Fubar | echo "Foo"
+
+menutranslate
+      \ &Foo\ bar
+      \ &Fubar | echo "Foo"
+
+menutranslate
+      \ &Foo\ bar
+      \ &Fubar| echo "Foo"
+
+menutranslate
+      \ &Foo\ bar
+      \ &Fubar
+      \ | echo "Foo"
+
+menutranslate &Foo\ bar
+      "\ comment
+      \ &Fubar | echo "Foo"
+
+menutranslate
+      "\ comment
+      \ &Foo\ bar
+      "\ comment
+      \ &Fubar | echo "Foo"
+
+menutranslate
+      \ &Foo\ bar
+      "\ comment
+      \ &Fubar| echo "Foo"
+      "\ comment
+
+menutranslate
+      "\ comment
+      \ &Foo\ bar
+      "\ comment
+      \ &Fubar
+      \ | echo "Foo"
+