Add abortSignal for createCompletion

[ngxson]

Resolves #19

Usage:

let wantToStop = false; // for example, it will change to true after a while

const outputText = await wllama.createCompletion(elemInput.value, {
  nPredict: 50,
  sampling: {
    temp: 0.5,
    top_k: 40,
    top_p: 0.9,
  },
  (token, piece, currentText, { abortSignal }) => {
    if (wantToStop) {
      abortSignal();
    }
  },
});