Add document_body and document_element to gloo_utils

[jplatte]

Summary

I would like to propose addition of document_body() and document_element() helper functions to gloo-utils.

Motivation

So projects wanting to access document.documentElement and document.body don't have to write their own helper functions calling the web_sys functions + unwrapping.

Detailed Explanation

pub fn document_body() -> HtmlElement {
    document().body().expect_throw("Can't find document body")
}

pub fn document_element() -> Element {
    document().document_element().expect_throw("Can't find document element")
}

Drawbacks, Rationale, and Alternatives

Drawbacks: -

Alternatives: -

Rationale / prior art: I have these helpers in caniuse.rs' code base: https://github.com/jplatte/caniuse.rs/blob/21cae646a489bec4537e6628492c1669a89221fb/src/util.rs#L15-L21

Unresolved Questions

-

[ranile]

I'd rename document_body to just body since it pulls the body element. That way it will be consistent with other functions.

I don't see why we shouldn't add those. If you want to PR those, feel free to.