Add proto2 required to generated field info

[timostamm]

To support field presence going forward, this PR adds a property to the minimal field info we generate.

For a proto2 message that defines required string string_field = 1, generated code changes as follows:

  static readonly fields: FieldList = proto2.util.newFieldList(() => [
-   { no: 1, name: "string_field", kind: "scalar", T: 9 /* ScalarType.STRING */ },
+   { no: 1, name: "string_field", kind: "scalar", T: 9 /* ScalarType.STRING */, req: true },

Previously, required fields were identified by the omission of opt: true. The new property will make it possible to merge the reflection-based serialization logic for proto2 and proto3.

When updating to the next release of @bufbuild/protobuf, it is important to update the plugin @bufbuild/protoc-gen-es as well, and regenerate code. (This PR does not rely on the new property req in generated code yet, but upcoming changes will.)

Before any changes were made, test coverage for field info was added in 202baf0.