v0.26.0 useBuiltIns Behavior change, style resolving & integrity support

We did it! Another release! 🥇

To upgrade:

1. Update your package.json file: set the @symfony/webpack-encore version to ^0.26.0.

2. Run yarn upgrade!

Changes: v0.25.0..v0.26.0

• [Behavior change] The Babel useBuiltIns option default value changed
  from entry to false, which means that polyfills may no longer be
  provided in the same way. This is due to a change in Babel and core-js.
  To get the same functionality back, run yarn add core-js --dev, then use:

  Encore.configureBabel(() => {}, {
      useBuiltIns: 'entry', // or try "usage"
      corejs: 3
  })

  This comes from #545 thanks to @Lyrkan.

• Added the ability to "resolve" CSS and Sass files without specifying
  the file extension and by taking advantage of the sass or style
  attribute in an npm package. For example, you can now import the main
  Bootstrap SASS file from within a SASS file by saying @import ~bootstrap.
  This will use the sass attribute from the bootstrap package.json
  file to find which file to load. #474 thanks to @deAtog.

• Added a new Encore.enableIntegrityHashes(), which will cause a new
  integrity key to be added to entrypoints.json with integrity values
  that can be included in the script or link tag for that asset - #522
  thanks to @Lyrkan.

• Allow some parts of configureBabel() to be used, even if there is
  an external .babelrc configuration file - #544 thanks to @Lyrkan.