This PR basically makes the `wasm{32,64}-unknown-unknown` targets work
_almost_ out of the box. All you need to do is enable Jiff's new `js`
crate feature. This will cause Jiff to depend on `js-sys` and
`wasm-bindgen`. Jiff will then use Javascript APIs to determine the
current time and time zone.
This PR also includes a new long form guide, `PLATFORM.md`, which
describes Jiff's platform support in one central location. (Most
information is already in Jiff's API docs, but it's scattered in a
variety of places.)
Finally, this adds a `wasm32-unknown-unknown` test to CI courtesy of
`wasm-pack`. It just does a basic sanity check that the current time and
time zone can be retrieved.
Fixes#56
