support strings in fs.readFile / fs.writeFile 'ascii' encoding (or add new encoding)

[SheetJSDev]

fs.readFile(path, 'ascii') resolves to an array of numbers, which is different from the other encoding types ('base64' and 'utf8' resolve to string). This also differs from react-native-fs (that library actually resolves to a binary string for the 'ascii' encoding). So to work with binary strings with RNFetchBlob, we would have to convert the bytes back to a string with a loop like:

readFile(file, 'ascii').then((res) => {
    const str = res.map(x => String.fromCharCode(x)).join("");
    // ...
});

Similarly, writing binary data becomes messy:

const data = str.split("").map(x => x.charCodeAt(0));
writeFile(file, data, 'ascii');

Would it be more efficient to perform the binary string conversion in native code? If so, would it make sense to change the behavior of 'ascii' or add a 'binary' encoding that deals with "binary strings"?

ping @itinance -- any thoughts? does it make sense to similarly add a 'binary' alias in RNFS?