Subpath exports for tree-shaking

[Jaydbrown]

Add ./indexer, ./batch, ./nonce subpath exports so consumers can import a slice without pulling the whole SDK.

[ZacLou]

I'd like to work on this issue. I have relevant experience with this tech stack and can deliver a clean, well-tested PR. I can submit within 1-2 days.

[ZacLou]

I'd like to work on this — opened a PR at #652.