Add finer control about worker creation

[cromefire]

Allows for better and clearer use, so that there is no implicit (hidden from the user) new Worker() happening.

[cromefire]

Usage in plain js:

const options = {
    workerConstructor() {
        return new Worker("path/to/file.js");
    },
    legacyWorkerConstructor() {
        return new Worker("path/to/file.js");
    },
    [...]
}

With worker-plugin:

const options = {
    workerConstructor() {
        return new Worker("./file.ts", {type: "module"});
    },
    legacyWorkerConstructor() {
        return new Worker("./file.ts", {type: "module"});
    },
    [...]
}