Feat: user defined user agent

[chaosharmonic]

What it sounds like, more or less.

I'm still figuring out how exactly the page-level bindings work, and this could maybe use some new tests, but in the meantime here's a sketch of how I'm envisioning the browser-level handling

[chaosharmonic]

Not exactly sure why this would be warning me about fmt on this many files when I changed two of them, but will figure that out when this is otherwise final