fix: throw when unsupported fields are detected

[achingbrain]

required is a field in proto2 but not in proto3 so throw if it is encountered while generating .ts from .proto.

It would be better to detect proto2 from the syntax directive but it's not in the output of the pbjs -t json command.

Fixes #34

[github-actions[bot]]

:tada: This PR is included in version protons-v6.1.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket:

[github-actions[bot]]

:tada: This issue has been resolved in version 3.0.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: