[BUG] PushDrop Data First

[sirdeggen]

Migrated from bsv-blockchain/ts-sdk#259

Bug Description

PushDrop.parse doesn't support the data first option of push drop scripts because it always expects the first push to be the locking pubkey.

Steps to Reproduce

Make a push drop token with lockPosition set to 'after'

Expected Behavior

We should add a parameter for lockPosition.

Actual Behavior

It will always fail because field data is interpreted as a pubkey and fails the DER check.

Error: Unknown point format
    at _Point.fromDER

[ty-everett]

Completed in PR #320. PushDrop supports and tests both data-first and lock-first positioning while preserving decode compatibility, including OP_16.