window -> self for Web Worker

[hata6502]

Thank you for maintaining sudachi-synonyms-dictionary and textlint-rule-no-synonyms!
By using self instead of window, it can supports both window and Web Worker.

https://developer.mozilla.org/ja/docs/Web/API/Window/self

[azu]

I think that self is similar one of window.
Probably, globalThis is suitable, but some browser does not support it yet.

• A horrifying globalThis polyfill in universal JavaScript · Mathias Bynens

Maybe, emurating all globals like this is best, but It's still unclear.

const getDictionaryURLFromGlobal = function () {
  if (typeof self !== 'undefined') { return selfsudachi-synonyms-dictionary; }
  if (typeof window !== 'undefined') { return windowsudachi-synonyms-dictionary; }
  if (typeof global !== 'undefined') { return globalsudachi-synonyms-dictionary; }
  throw new Error('unable to locate global object');
};

webpack can do workaround like it, but webpack has not suitable option.

• Add target: "universal" · Issue #6525 · webpack/webpack