feat: implement createSinglebyteEncoder

[ChALkeR]

Ref: #9
Doesn't yet solve that, but a step towards encoders

[Copilot]

Pull request overview

This PR implements the createSinglebyteEncoder function to provide encoding functionality for single-byte character encodings, complementing the existing decoder functionality. It adds support for encoding strings to bytes according to the WHATWG Encoding standard's legacy single-byte encodings.

• Implements createSinglebyteEncoder() with optimized paths for Node.js and fallback implementations
• Adds comprehensive test coverage for encoder functionality including ASCII subset validation and error cases
• Updates documentation with API details for the new encoder functions

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/single-byte.test.js	Adds encoder tests to validate encoding for ASCII subsets, encoding indices, and x-user-defined encoding with error cases
single-byte.node.js	Implements Node.js-optimized encoder using Buffer and Uint16Array for performance, with ASCII fast path
single-byte.js	Implements fallback encoder with native encoder support and optimistic encoding strategy
fallback/single-byte.js	Adds encodeMap() function to generate reverse lookup maps for encoding codepoints to bytes
benchmarks/windows1252.bench.js	Adds benchmark tests comparing encoder performance across libraries for ASCII and complex strings
README.md	Documents the new createSinglebyteEncoder() and windows1252fromString() functions with usage examples

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[Copilot]

The description says "that decodes bytes to a string" but this function encodes a string to bytes. The description should say "that encodes a string to bytes".

Suggested change

	Returns a function `encode(string)` that decodes bytes to a string.
	Returns a function `encode(string)` that encodes a string to bytes.

[Copilot]

Typo: "codepoins" should be "codepoints".

Suggested change

	Will throw on non well-formed strings or any codepoins which could not be encoded in `windows-1252`.
	Will throw on non well-formed strings or any codepoints which could not be encoded in `windows-1252`.