fix: helia-401 - add browser Helia example

[TheGreatAlgo]

As requested here: https://github.com/ipfs/helia/pull/103#discussion_r1178568734

This is an example for a Helia client that can be loaded in the browser.

[achingbrain]

Thanks loads for opening this, it's really helpful.

It's missing a test though - can you please add one to ensure there are no regressions in future. You can crib from the existing browser app tests in this repo, they show how to open a browser, navigate to a page, etc.