Allow for specifying the port

Currently `dev-harness` only runs on port `8080` and can't be changed. This is annoying when trying to develop two plugins side-by-side, or if port `8080` is already used by something else.

Webpack already allows specifying the port (e.g. `npm start -- --port 1234`), but `8080` is hardcoded into the Adapter.

```jsx

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