fix: make base32 decode case-insensitive

[sumanjeet0012]

Fixes #36

Description

This pull request makes Base32StringConverter case-insensitive during decoding, which complies with the multibase specification and matches the behavior of the Go reference implementation (go-multibase).

Previously, decoding a Base32 payload with mismatched case compared to its prefix encoding (e.g. base32 with an uppercase payload bMZXW6, or base32upper with a lowercase payload Bmzxw6) would raise a ValueError during alphabet mapping.

Changes Made

• Modified Base32StringConverter.decode in multibase/converters.py to check self.digits.islower() and self.digits.isupper(). It now normalizes the payload's case to match the alphabet before iterating over the bytes.
• Added explicit unit tests (test_decode_base32_case_insensitive) to tests/test_multibase.py to verify this behavior for base32, base32upper, base32pad, and base32hex.

Verification

• Confirmed the newly added tests successfully pass (pytest tests/).
• Verified all other pre-existing 266 tests pass.
• Ensured make lint catches no errors and runs completely successfully.