ContextProvider and registry, reorg exports

Changes:

1. Adds ContextProvider, a static registry that

• Is a central point for obtaining Context implementations.
• Auto-registers the FFI implementation if required, when running under Deno.
• Allows additional implementations of Context to be registered, ready for card-emulator, node-compat, etc

2. Reorganized exports to facilitate tree-shaking,

• mod.ts - exports complete functionality
• iso7816/iso7816.ts exports ISO7816 types, enums and classes, non-Deno specific
• pcsc/pcsc.ts exports PC/SC types and enums, non-Deno specific
• deno-pcsc-ffi/deno-pcsc-ffi.ts exports Deno specific 'ffi' functionality