Inject metadata/extra fields into the build *.tgz file

[mStirner]

const insertExtraFields = (data, fields) => {
    let fullLength = 0;
    for (const field in fields) {
        const si2 = field >> 8;
        if (!si2 || si2 > 255) {
            throw new Error('The extra IDs must be greater than 255 and less than 65536');
        }
        fullLength += fields[field].length + 4;
    }
    if (fullLength > 65535) {
        throw new Error(
            'The total size of the extra header is '
            + fullLength
            + ' bytes, but it must be at most 65535 bytes'
        );
    }
    const afterExtra = data.subarray(10);
    const out = new Uint8Array(12 + fullLength + afterExtra.length);
    out.set(data.subarray(0, 10));
    out.set(afterExtra, out.length - afterExtra.length);
    // Enable FEXTRA
    out[3] |= 4;
    out[10] = fullLength & 255;
    out[11] = fullLength >> 8;
    let b = 12;
    for (const field in fields) {
        out[b] = field & 255;
        out[b + 1] = field >> 8;
        const ext = fields[field];
        out[b + 2] = ext.length & 255;
        out[b + 3] = ext.length >> 8;
        out.set(ext, b + 4);
        b += ext.length + 4;
    }
    return out;
}

const asciiToID = str => {
    if (str.length != 2) throw new TypeError('extra ID must be two characters long');
    return str.charCodeAt(0) | (str.charCodeAt(1) << 8);
  }

module.exports = {
    insertExtraFields,
    asciiToID
};

https://github.com/OpenHausIO/playground/tree/main/plugin-wrapper-tgz-meta-data
https://github.com/OpenHausIO/playground/tree/main/tgz-plugin-creation
101arrowz/fflate#82

Before implementing that, we should ask for intents from the wizard (#9) and save metadata.