V2: Introduce DescMessage.field

[timostamm]

In some situations, it's useful to be able to refer to a field by name.

DescMessage provides all fields in the property fields: DescField[] - that's convenient to loop through all fields, but it's awkward to pick a field by name.

This PR adds another property to DescMessage:

/**
 * All fields of this message by their "localName".
 */
readonly field: Record<string, DescField>;

In generated code, the property is a type-safe record. You get autocomplete for field names, and a typo is a compile-time error:

import { UserDesc } from "./gen/example_pb.js";

UserDesc.field.firstName; // DescField
UserDesc.field.firstNam; // type error

This PR also changes the signatures of isFieldSet and clearField to make use of this new feature. Instead of having to pass in the message descriptor, the message, and the field name, both functions only require two arguments now: The message, and the field:

import { create, clearField } from "@bufbuild/protobuf";
import { UserDesc } from "./gen/example_pb.js";

const user = create(UserDesc);
clearField(user, UserDesc.field.firstName);

[timostamm]

We're eagerly populating the record - it's quite simple and cheap.

[timostamm]

That's a nice practical example of where this feature can be useful. I imagine it will also be useful to construct FieldMasks.

[timostamm]

Same as before, a foreign field is never set, and clearing a foreign field is a no-op.