Consider modularizing SEP implementations away from the rest of the SDK

[Shaptic]

Describe the problem It's difficult to interact with ecosystem-level agreements (aka SEPs) in the JS SDK. For example, the entirety of SEP-10 is lumped into utils.ts. This makes it difficult to reason about, because it's hard to distinguish between what's relevant to the SDK at large, what's relevant to a specific SEP, and what's relevant to SEPs in general.

Describe the solution you'd like Per-SEP modules that isolate implementations and helpers.

[leighmcculloch]

@ire-and-curses @bartekn The Go SDK would also benefit from this idea because it has its SEP-10 functions bundled in the txnbuild package.