feat: add ArrayBuffer output support

[ChALkeR]

Resolves #65

[Copilot]

Pull request overview

Adds first-class 'arraybuffer' output support across the bytes encoding/decoding APIs, aligning runtime behavior, TypeScript declarations, and test coverage to resolve #65.

Changes:

• Extend output format handling to support returning ArrayBuffer (runtime + typings).
• Update TypeScript overloads across utf8/hex/base32/base64/base58/base58check/bigint modules to include 'arraybuffer'.
• Expand test suites to validate 'arraybuffer' output across affected modules.

Reviewed changes

Copilot reviewed 9 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
fallback/_utils.js	Adds 'arraybuffer' handling to fromUint8 / fromBuffer conversion helpers.
array.d.ts	Expands OutputFormat to include 'arraybuffer'; adjusts typed array helper signatures accordingly.
utf8.d.ts	Adds 'arraybuffer' overloads/return unions for UTF-8 encoding functions.
hex.d.ts	Adds 'arraybuffer' overload/union return type for fromHex.
bigint.d.ts	Adds 'arraybuffer' overload/union return type for fromBigInt.
base64.d.ts	Adds 'arraybuffer' overloads/union return types for base64 decoders.
base32.d.ts	Adds 'arraybuffer' overloads/union return types for base32 decoders.
base58.d.ts	Adds 'arraybuffer' overloads/union return types for base58 decoders.
base58check.d.ts	Adds 'arraybuffer' overloads/union return types for base58check decoders (async + sync).
tests/utf8.test.js	Adds 'arraybuffer' to invalid-format coverage and validates 'arraybuffer' output roundtrips.
tests/utf8.lib.test.js	Mirrors UTF-8 'arraybuffer' coverage for library import path.
tests/hex.test.js	Adds 'arraybuffer' decoding assertions and fixtures pool tracking.
tests/bigint.test.js	Adds 'arraybuffer' format assertions for fromBigInt.
tests/base64.test.js	Adds 'arraybuffer' format assertions for base64/base64url decoders.
tests/base32.test.js	Adds 'arraybuffer' format assertions for base32/base32hex decoders.
tests/base58.test.js	Adds 'arraybuffer' format assertions and adjusts non-pooled tracking exclusions.
tests/base58check.test.js	Adds 'arraybuffer' format assertions and adjusts non-pooled tracking exclusions.

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