Mark private exports @private

[timostamm]

The @bufbuild/connect package exports many types from subpath exports. Nearly all of them are for internal use, and only exported because we need them in the @bufbuild/connect-web package to provide gRPC-web and Connect Transports with minimal bundle size.

It is not feasible for us to support the huge API surface with the same stability promise as the main package exports. This PR marks all internal code with the @private JSDoc annotation. The index.ts files gives an explanation:

// All exports below are private — internal code that does not follow semantic
// versioning.
// We will try hard to avoid breaking changes, but if you depend on the
// following exports, we recommend that you do so with an exact version
// constraint (no ~ or ^).

Exports that are necessary to support other platforms and server frameworks are not marked private. createTransport() and createHandlerFactory() from @bufbuild/protocol-connect and the other protocol subpaths, and the types for universal handlers and clients from @bufbuild/protocol are still part of the public API.