patch 9.1.1942: Vim9: Assignment to read-only registers @: and @% is allowed

patch 9.1.1942: Vim9: Assignment to read-only registers @: and @% is allowed

Problem:  Assignment to read-only registers @: and @% is allowed during
          compilation.
Solution: Abort compilation and emit an E354 error when assigning to
          these registers (Doug Kearns).

Fix the E354 error emitted when attempting to declare @: with :var so
that it references the correct register, @:,  rather than the garbage
string "^@".

closes: #18806

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