Expose experimental syntax "editions" in descriptor sets

[timostamm]

This change exposes the editions syntax in a DescriptorSet, the wrapper around protobuf file descriptor messages provided by createDescriptorSet from @bufbuild/protobuf.

The type DescFile receives a new property:

  /**
   * The edition of the protobuf file. Will be EDITION_PROTO2 for syntax="proto2",
   * EDITION_PROTO3 for syntax="proto3";
   */
  readonly edition: Omit<Edition, ...>;

The syntax property is updated to include "editions" in the union type:

  /**
   * The syntax specified in the protobuf source.
   */
  readonly syntax: "proto3" | "proto2" | "editions";

For example, a file with syntax="proto3" will have the properties syntax: "proto3" and edition: Edition.EDITION_PROTO3. A file with edition="2023" will have the properties syntax: "editions" and edition: Edition.EDITION_2023.

Note that this change may cause compile time errors in your code, even if you are not using editions yet.

Edition FeatureSets are not exposed yet. protoc-gen-es cannot generate code for editions yet. This change just allows createDescriptorSet to wrap descriptors using editions.

[jhump]

nit: I would probably unify this with "proto3" so it is at least closer to accurate. By default in editions (without element-level feature override), it behaves the same as proto3.

[timostamm]

In EDITION_2023 it does, but in EDITION_PROTO2 it does not 😛 Will update this logic in the next step, where this value will be plucked from FeatureSetDefaults.

[jhump]

But if the edition is EDITION_PROTO2, the syntax will not be "editions"; it will be "proto2".

[timostamm]

You're right about "proto2" of course.

Here is the new implementation of isPackedFieldByDefault. It uses the seeded default value of repeated_field_encoding for the edition, including EDITION_PROTO2, EDITION_PROTO3, EDITION_2023, and any future edition.

The implementation is shorter and seems less ambiguous. I guessed it would, which is why I wanted to hold off modifying it in this PR. I should have explained better.