std: add mem.SplitIterator.peek()

[nektro]

std.mem.TokenIterator already has a method by this name

[squeek502]

This will need to be rebased now that #15579 has been merged

The updated implementation to handle the other delimiter types could be something like:

pubfnpeek(self: *Self) ?[]constT {
conststart=self.indexorelsereturnnull;
constend=if (switch (delimiter_type) {
.sequence=>indexOfPos(T, self.buffer, start, self.delimiter),
.any=>indexOfAnyPos(T, self.buffer, start, self.delimiter),
.scalar=>indexOfScalarPos(T, self.buffer, start, self.delimiter),
}) |delim_start|delim_startelseself.buffer.len;
returnself.buffer[start..end];
}