Add WebJar class for accessing WebJar files

[valencik]

Closes #633

There's basically two features of this work:

• We only need to specify the WebJar version once in the using dep directive, this makes manually bumping easier.
• We reduce the boilerplate of loading a file from a WebJar

---

Felt weird doing all this without fs2 or IO, but so far none of our uses of this are in IO.
The whole org.graalvm.polyglot.Context stuff feels like it could have a cool little micro library around it.

[armanbilge]

I feel like the ClassLoader should be a property of the WebJar class, so that the same one is used consistently each time (and it's not at the whim of the thread it's running on).

[valencik]

Makes sense. I've replaced these two bits of code with:

private val classLoader = classOf[WebJar].getClassLoader