runtime(vim): Update base-syntax, disallow '.' at start of menu item names (#14232)

runtime(vim): Update base-syntax, disallow '.' at start of menu item names (#14232)

Disallow '.' at the start of a menu item name.

This is the menu path separator character and should be escaped with a
'\' in this case.

Partially fixes #14230.  "popup" is still incorrectly matched as the Ex
command.

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