patch 9.2.0768: legacy/vim9cmd modifiers are not exclusive

patch 9.2.0768: legacy/vim9cmd modifiers are not exclusive

Problem:  legacy/vim9cmd modifiers are not exclusive
Solution: when parsing command modifiers clear the other flag
          (Illia Bobyr)

When one of the ":legacy" or ":vim9cmd" prefixes is used, it sets a
flag.  As the rest of the code might check either of the flags, setting
both could produce confusing results.  It does not make a lot of sense
to use both prefixes in the same command.  But just in case it is used,
selecting the last prefix seems reasonable.

closes: #20682

Signed-off-by: Illia Bobyr <illia.bobyr@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>