transpile to ESM with tsdown

[dynst]

index.ts and json-safe.ts are actually empty of any runtime code, they only contain types, so no sourcemap files are generated for them.

[dynst]

tsdown is needed because if you try to use tsc to transpile to ESM it will simply choke on every extensionless import in the codebase:

cosmjs-types/src/cosmos/tx/v1beta1/tx.ts

Line 2 in db4cbb1

import { Any } from "../../../google/protobuf/any";

Is there anything blocking this PR?