Implement SelectorsAPI

[robingustafsson]

Implement a Selectors API, to allow for registering of custom selector engines besides the ones provided out-of-the-box. This can for example be useful to allow implementions of selector engines for specific frontend frameworks like React (see docs for Playwright) or Vue (see docs for Playwright).

Relevant links:

• Playwright docs: https://playwright.dev/docs/api/class-selectors