chore: add run:browser task

[tjjfvi]

Allows testing the various examples in the browser via e.g.

deno task run:browser examples/transfer.ts

The task file is a bit hacked together, but as it's an internal util, I think this is fine.

[tjjfvi]

To clarify:

Users who want to use capi from the browser will not use anything like run_browser – they will likely use the DNT-produced package along with a bundler of their choice.

The run_browser script is solely for internal testing, so that we can run the Deno example scripts in the browser without having to go through DNT and a bundler.