ts: measure text with offline canvas

measureText.ts

const canvasContext = document
  .createElement("canvas")
  .getContext("2d") as CanvasRenderingContext2D;

/**
 * measures text with the [canvas API](https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/measureText)
 * [avoiding expensive reflows](https://gist.github.com/Yukiniro/876826e1450b1f8cf755d2cea83cda65).
 */
export function measureText(text: string, font: string = "normal 16px serif") {
  canvasContext.font = font;
  return canvasContext.measureText(text);
}