Although our API had a place to provide a start position for scanning, it
didn't actually work in practice because the scanner wasn't aware of it
and so it would immediately undo the effect of that start offset when
making the first position adjustment.
Now we'll remember the byte offset we started at and offset the indices
the generate scanner produces so that they are are treated as relative
to that start byte instead of byte zero.
Since we rarely start with a non-zero pos this doesn't affect much, but
one specific thing it affects is the positions of native syntax templates
inside JSON syntax strings.
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