Handle reserved field names & numbers when they're nested inside groups

[Asgavar]

Currently, trying to parse a .proto file with such construct results in an error:

Error: illegal token 'reserved'

However, there's nothing in the Protocol Buffers specification[1] that would suggests that it's illegal, moreover, even Google itself does use it, e.g. in their Chromium Sync protocol[2].

[1] https://developers.google.com/protocol-buffers/docs/reference/proto3-spec#reserved [2] https://chromium.googlesource.com/chromium/src.git/+/refs/heads/master/components/sync/protocol/sync.proto#848