move file parsing to single-pass static methods

[EliotJones]

for the file 0002973.pdf in the test corpus we need to completely overhaul how initial xref parsing is done since we need to locate the xref stream by brute-force and this is currently broken. i wanted to take this opportunity to change the logic to be more imperative and less like the pdfbox methods with instance data and classes.

currently the logic is split between the xref offset validator and parser methods and we call the validator logic twice, followed by brute-force searching again in the actual parser. we're going to move to a single method that performs the following steps:

1. find the first (from the end) occurrence of "startxref" and pull out the location in bytes. this will also support "startref" since some files in the wild have that
2. go to that offset if found and parse the chain of tables or streams by /prev reference
3. if any element in step 2 fails then we perform a single brute-force over the entire file and like pdfbox treat later in file-length xrefs as the ultimate arbiter of the object positions. while we do this we potentially can capture the actual object offsets since the xref positions are probably incorrect too.

the aim with this is to avoid as much seeking and re-reading of bytes as possible. while this won't technically be single-pass it gets us much closer. it also removes the more strict logic requiring a "startxref" token to exist and be valid, since we can repair this by brute-force anyway.

we will surface as much information as possible from the static method so that we could in future support an object explorer ui for pdfs.

this will also be more resilient to invalid xref formats with e.g. comment tokens or missing newlines.

[BobLd]

@EliotJones don't hesitate if you don't agree with my feedback, or if you need help

[EliotJones]

@BobLd all good I just haven't had time to get back to this yet. I also need to check how it handles the case where the file contains object streams.

[EliotJones]

@BobLd I've handled a couple of feedback items now and added the test I wanted to include. Let me know if you have any further feedback.