Proptests for the query parser

[V0ldek]

Is your feature request related to a problem? Please describe.
We should use the proptest crate to comprehensively test the query parser.

Describe the solution you'd like
As usual for proptests, this is a bit of a complex design space.

It's relatively easy to create proptests for correct query strings – just generate an arbitrary sequence of selectors and stringify them. It might be good to start with that as a single PR first.

Query strings that should result in an error are a bit more tricky. There are many things that can cause a parsing error, and just generating random garbage won't give too good a coverage. We can start by generating a sequence of valid selectors interspersed with "error" segments, and then figuring out smart kinds of "error" – things like three-dot sequences, root selectors inside unescaped labels...

As usual for proptests, we need to do it iteratively. Start with simpler tests and then consequently add more complex cases.

Additional context
I'm not sure what the acceptance criteria for this issue are, so we're just gonna wing it – we'll close it when we feel sufficiently happy with the test suite for something labelled as "1.0.0".

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[V0ldek]

We have very good proptests and a fuzzer for valid queries. Adding proptests for error cases might be way too hard and not bring that much benefit. Snapshot testing for errors would be better - at least one test for each error kind in rsonpath-syntax.