Document the Context API and the helper function

[lilnasy]

Basic Usage

import { defineContext, defineExtension } from "es-codec"

interface Logger {
    log(...args : any[]): void
}

const { encode, decode } = defineContext<Logger>().createCodec([
    defineExtension({
        name: "URL",
        when(x): x is URL { return x.constructor === URL },
        encode(url, context) {
            context.log("encoding url", url)
            return url.href
        },
        decode(href, context) {
            context.log("decoding url", href)
            return new URL(href)
        }
    })
])

const encodedUrl = encode(new URL("https://example.com"), console)
const decodedUrl = decode(encodedURL, console)