Update dependency sass-loader to v8

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
sass-loader	devDependencies	major	7.3.1 -> 8.0.0

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Release Notes

webpack-contrib/sass-loader

### [`v8.0.0`](https://togithub.com/webpack-contrib/sass-loader/blob/master/CHANGELOG.md#​800httpsgithubcomwebpack-contribsass-loadercomparev731v800-2019-08-29) [Compare Source](https://togithub.com/webpack-contrib/sass-loader/compare/v7.3.1...v8.0.0) ##### ⚠ BREAKING CHANGES - minimum required `webpack` version is `4.36.0` - minimum required `node.js` version is `8.9.0` - move all sass (`includePaths`, `importer`, `functions`) options to the `sassOptions` option. The `functions` option can't be used as `Function`, you should use `sassOption` as `Function` to achieve this. - the `data` option was renamed to the `prependData` option - default value of the `sourceMap` option depends on the `devtool` value (`eval`/`false` values don't enable source map generation) ##### Features - automatically use the `fibers` package if it is possible ([#​744](https://togithub.com/webpack-contrib/sass-loader/issues/744)) ([96184e1](https://togithub.com/webpack-contrib/sass-loader/commit/96184e1)) - source map generation depends on the `devtool` option ([#​743](https://togithub.com/webpack-contrib/sass-loader/issues/743)) ([fcea88e](https://togithub.com/webpack-contrib/sass-loader/commit/fcea88e)) - validate loader options ([#​737](https://togithub.com/webpack-contrib/sass-loader/issues/737)) ([7b543fc](https://togithub.com/webpack-contrib/sass-loader/commit/7b543fc)) - reworked error handling from `node-sass`/`sass` - improve resolution for `@import` (including support `_index` and `index` files in a directory) ##### Bug Fixes - compatibility with `pnp` ##### [7.3.1](https://togithub.com/webpack-contrib/sass-loader/compare/v7.3.0...v7.3.1) (2019-08-20) ##### Bug Fixes - minimum `node` version in `package.json` ([#​733](https://togithub.com/webpack-contrib/sass-loader/issues/733)) ([1175920](https://togithub.com/webpack-contrib/sass-loader/commit/1175920))

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