regex: Allow any single char to be SIMPLE
This commit relaxes the previous requirement that an EXACTish node must
contain a single Latin1-range character in order to be considered
SIMPLE. Now it allows any single character, not just Latin1.
This allows above-Unicode characters to be in optimizations like STAR or
CURLY, instead of having to match with the more complex CURLYM; and it
brings EXACTish nodes in alignment with other SIMPLE nodes, such as
those matching \w or the dot metacharacter, which all along have
supported any code point being SIMPLE