Make webpack autoupload plugin compatible with webpack 5

[anthmatic]

Description and Context

Between webpack v4 > v5, a couple of breaking changes were released that affected this plugin:

• data was removed from assets, specifically emitted and existsAt
• In v4, files were always emitted on the first compile, in v5 they're only emitted if they've changed

This pr:

• removes the dependency on existsAt and builds the filepath manually
• checks for the presence of emittted or compares an asset against the emittedAssets set
• keeps track of the first compile to ensure that we continue to upload correctly

Fixes #408

[miketalley]

Code looks good and it works in testing with webpack versions 4 and 5!

The only thing was the upload of all assets output by the webpack build when a single file changes (as we discussed). LMK how I can help with testing this more if needed!

[anthmatic]

After some testing (thanks @miketalley!) I reworked the logic here to make sure that we're only uploading emitted assets and also ensure that the first compile uploads all files consistently. I updated the PR description as well.

FYI I'm planning to get this out in a new release, but from this new repo https://github.com/HubSpot/webpack-cms-plugins