V2: Add DescEnum.value

[timostamm]

Similar to DescMessage.field, which allows lookup of field descriptors by local name, this adds DescEnum.value to allow lookup of enum value descriptors by numeric value:

import type { DescEnumValue } from "@bufbuild/protobuf";
import type { PhoneType, PhoneTypeSchema } from "./gen/example_pb";

const value: DescEnumValue = PhoneTypeSchema.value[PhoneType.MOBILE];