Replace percy-webapis with web-sys

[chinedufn]

Now that web-sys has matured we shouldn't be rolling our own web apis. We'll need to add the web-sys dependency

[dependencies.web-sys]
git = "https://github.com/rustwasm/wasm-bindgen"
features = [
    "Window",
    # ...
]

And then one by one replace our percy_webapis crate usages with web-sys