Validate usage of features in editions

[jhump]

This adds lots of validation of feature values, to mirror the logic performed in protoc.

This also adds some other validation checks that are implemented in protoc but were not previously implemented here in protocompile.

This would have been the last change needed to support editions in protocompile, except it looks like we'll also need to support extra validation of which features are allowed in which editions (see this protoc commit that just landed this week: protocolbuffers/protobuf@b09b3e4).

Since that change includes new stuff in descriptor.proto which isn't likely to make it into protobuf-go's descriptorpb package until v27 is released (or at least until the first release candidate is made available), we'll need to provide a stop-gap here to opt into editions support. I was hoping to just enable editions support in this repo once the work was complete; but now it won't be complete until after the above happens, yet we need to start working with the support that has been implemented so far in order to update logic in the buf CLI. So we'll have to expose the current internal flag to allow importing code (like the buf CLI) to opt into working with editions so we can implement and test editions-related things there.

[jhump]

For context, here are the checks in the protoc C++ code base:

1. Validate file options: https://github.com/protocolbuffers/protobuf/blob/d4525cb9edb196e3c52e75c0e4e88f14891b0c6f/src/google/protobuf/descriptor.cc#L7546
2. Validate file-level features: https://github.com/protocolbuffers/protobuf/blob/d4525cb9edb196e3c52e75c0e4e88f14891b0c6f/src/google/protobuf/descriptor.cc#L7836
3. Validate field optoins: https://github.com/protocolbuffers/protobuf/blob/d4525cb9edb196e3c52e75c0e4e88f14891b0c6f/src/google/protobuf/descriptor.cc#L7650
4. Validate field-level features: https://github.com/protocolbuffers/protobuf/blob/d4525cb9edb196e3c52e75c0e4e88f14891b0c6f/src/google/protobuf/descriptor.cc#L7855

Notably different is how this ValidateOptions for files also does the proto3 syntax checks. But those aren't present here because those don't require interpreting options to perform, so protocompile does those much earlier, at the point of producing descriptor protos from the AST in the parser sub-package. The checks in this PR are all after options are interpreted.

One exception: this PR resurrects a check that is done in the parser, related to proto3 and enum values starting at zero. In a prior PR (#260), that check was removed in favor of the later check, which can also work with editions (which can't be done until after options are interpreted since features in editions are options). But in this PR, I've restored the check (very first commit) so that we can provide a more precise error message as well as detect the problem earlier.

[emcfarlane]

lgtm!