V2: Use suffix "Schema" for generated descriptors

[timostamm]

When we generate code for a Protobuf message, we generate a TypeScript type for the instance shape, and a const for the message descriptor.

While it's possible to use the same identifier for both, the ambiguity isn't ideal. So far, we've been adding a "Desc" suffix to the name of the generated descriptor to differentiate:

syntax = "proto3";

message User {
  // ...
}

Generated:

export type User = ...

export const UserDesc = ...

// usage:
toBinary(UserDesc, user);

This PR changes the suffix of the descriptor to "Schema". It's a much more recognizable name:

export type User = ...

export const UserSchema = ...

// usage:
toBinary(UserSchema, user);

Related:

• This change also applies to enum descriptors.
• For file descriptors, their prefix is simplified from "fileDesc_" to "file_".
• In @bufbuild/protoplugin, GeneratedFile.importDesc is renamed to importSchema.

[timostamm]

This seems more understandable for users who aren't familiar with Protobuf descriptors.