Add publicPath support to webpack-plugin

[LukeSheard]

Support webpack's publicPath option and add long term caching options into the webpack plugin.

Changes:

• 42dec1d - update webpack example to be a cross origin asset application
• 84d8536 - renames the loader filenames for clarity of their real intention
• fac6af1 - fixes the bug with locating the worker and wasm assets paths when loaded with a publicPath
• 2e1adfd - adds support for long term caching into the output of the perspective loaders.
• bc3585c - updates the webpack example to utilize the long term caching webpack options

[texodus]

This causes duplicate hashed webworker assets to be output in the Jupyterlab plugin on every build

[texodus]

Thanks for the PR!

Thoroughly tested, fixed one minor regression, but everything else looks good!