Bug fixes & updating dependencies

What's better than Encore on Webpack 4? Encore on Webpack 4 with a few bugs squashed!

Highlights:

• #439 updating resolve.extensions to include webpack4 default extensions
• #440 updating css-loader dep and ts-loader dev dep
• #441 Appending the _tmp_shared contents via an earlier hook
• #451 pretty-printing entrypoints.json

Happy Webpacking!