Update how loading pages work

[patrick91]

Current we always re-execute the code when the "file" changes, and when we do that we set a variable called browser_page to the URL we defined in the preview browser.

We should change this to have our javascript calling a function to load a page, its signature could look like this:

def load_page(url, method, body) -> Response:
    ...

this will allow us to also handle form events and other kinds of navigation.