Improve type representation for open enums

[timostamm]

Protobuf has closed and open enums. A closed enum (proto2) only accepts declared values, while an open enum (proto3) also accepts other values. protoc-gen-es generates TypeScript enums, which are always closed since version 5.0.

This means that generated types do not match the semantics for open Protobuf enums. There is no indication that an enum field could contain a value that isn't one of the TypeScript enum values, and additional checks might even be flagged as unnecessary or incorrect by the compiler.

We should investigate how to improve the generated enum types or fields to properly reflect open enum semantics.

Context:

• protobuf.dev on Enum behavior
• Update FAQ entry on TypeScript enums vs. string unions #1082
• Add the open/closed behavior for enums to documentation #522
• Feature Request: enumAsConst option #1139