runtime(colors): color names in the v:colornames dict should be lower cased

runtime(colors): color names in the v:colornames dict should be lower cased

Vim will lookup color names from the v:colornames dictionary by its
lower case color name. So when sourcing the v:colornames dictionary,
make sure to convert upper case color names to lower case.

Also, while at it, mention in the documentation, that the dictionary
should contain lower case names only.

fixes: #13976
Signed-off-by: Christian Brabandt <cb@256bit.org>