fix unicode indexing in parse(Complex, string) (#51758)
This fixes a string-indexing bug introduced in #24713 (Julia 0.7).
Sometimes, this would cause `parse(Complex{T}, string)` to throw a
`StringIndexError` rather than an `ArgumentError`, e.g. for
`parse(ComplexF64, "3 β+ 4im")` or `parse(ComplexF64, "3 + 4αm")`. (As
far as I can tell, it can never cause parsing to fail for valid
strings.)
The source of the error is that if `i` is the index of an ASCII
character in a string `s`, then `s[i+1]` is valid (even if the next
character is non-ASCII) but `s[i-1]` is invalid if the previous
character is non-ASCII.
