jiff: add robust WASM support

[BurntSushi]

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

[BurntSushi]

For posterity, I had this conversion with the wasmtime folks about how best to approach WASM support. Their help was instrumental in figuring out what to do here, and also in helping to educate me enough to write some half way decent WASM platform documentation.