feat: expose code property on Encoder class

[sumanjeet0012]

Fixes #47

Description

This pull request adds a public .code property to the Encoder class.

Changes

• Exposes self._codec.code via a read-only property code.
• Brings py-multibase into alignment with go-multibase, which exposes the encoding code via Encoder.Encoding().
• Added test validation to ensure encoder.code returns the correct multibase prefix byte.

[acul71]

Request changes — missing newsfragment (blocker)

Thanks for this clean, focused fix for #47. The Encoder.code property matches the issue proposal and looks correct; CI and local make lint / typecheck / test / docs-ci all pass.

Blocker

Please add a towncrier newsfragment:

• Path: newsfragments/47.feature.rst
• Content (example): Added a public ``Encoder.code`` property exposing the multibase prefix byte.
• Must end with a trailing newline

Newsfragments are mandatory for approval on this repo.

Also recommended (non-blocking)

• Rebase onto current origin/master (branch is ~6 commits behind; dry-run merge is clean today).
• Note overlap with open PR #54 on the same files — coordinate merge order.
• Optional: assert another encoding for .code (e.g. base16 → b"f") and/or mention encoder.code in the README Encoder example.

Happy to re-review once 47.feature.rst is added.