Mostly not having to list version-specific excludes when testing against ripper/parse.y

Also don't test new syntax additions against the parser gems. The version support
for them may (or may not) be expanded but we shouldn't bother while the ruby version
hasn't even released yet.
(ruby_parser translation is not versioned, so let as is for now)

I also removed excludes that have since been implemented by parse.y

https://github.com/ruby/prism/commit/e5a0221c37

This one has been on my mind for a while now.

Currently, there are only tests against the latest syntax version.

This changes the snapshot structure as follows:
* Snapshots at their current location are tested against all syntax versions
* Snapshots inside a version folder like "3.3" are tested against all versions starting from that version
* Snapshots inside a version folder like "3.3-4.2" are tested against all versions in the given range.

This makes sure that as new syntax is added, older versions still work as expected.
I also added a few tests for now valid syntax that should be invalid in older versions (and the other way around as well)

These tests run really fast. So even though it does 3x the work for these, I am still able to run the whole test suite in just 11 seconds.

https://github.com/ruby/prism/commit/5191b1aa68

Skipping detecting the encoding is almost always right, just for binary it should actually happen.

A symbol containing escapes that are invalid
in utf-8 would fail to parse since symbols must be valid in the script encoding.
Additionally, the parser gem would raise an exception somewhere during string handling

https://github.com/ruby/prism/commit/fa0154d9e4

https://github.com/ruby/prism/commit/6f886be0a4