feat: add setMeasureFunction for custom measurement backends

[intergalacticspacehighway]

Why?

• The idea is borrowed from Yoga which has a concept of measure function for measureable nodes (Text). This allows us to plug pretext into non browser environments like React Native.

How?

Exported setMeasureFunction function which can be used to provide a custom measure function.

• Example of using pretext in React native environment with @shopify/react-native-skia and custom native measurement module - https://github.com/intergalacticspacehighway/pretext-react-native-example/

Test plan

• Added basic JS tests with measure function.
• Test React native example - https://github.com/intergalacticspacehighway/pretext-react-native-example/

Alternatives

Alternate solution for cross platform would be to polyfill the OffscreenCanvas. This also works, can just be documented.

globalThis.OffscreenCanvas = class {
  getContext() {
    let currentFont = '';
    return {
      set font(f: string) { currentFont = f; },
      get font() { return currentFont; },
      measureText(text: string) {
        return { width: MyMeasureFunction() };
      }
    };
  }
};

[intergalacticspacehighway]

React native has a navigator object but no userAgent string 😅