Update dependency file-loader to v4

[renovate-bot]

This PR contains the following updates:

Package	Type	Update	Change
file-loader	devDependencies	major	3.0.1 -> 4.2.0

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Release Notes

webpack-contrib/file-loader

### [`v4.2.0`](https://togithub.com/webpack-contrib/file-loader/blob/master/CHANGELOG.md#​420httpsgithubcomwebpack-contribfile-loadercomparev410v420-2019-08-07) [Compare Source](https://togithub.com/webpack-contrib/file-loader/compare/v4.1.0...v4.2.0) ##### Features - `postTransformPublicPath` option ([#​334](https://togithub.com/webpack-contrib/file-loader/issues/334)) ([c136f44](https://togithub.com/webpack-contrib/file-loader/commit/c136f44)) ### [`v4.1.0`](https://togithub.com/webpack-contrib/file-loader/blob/master/CHANGELOG.md#​420httpsgithubcomwebpack-contribfile-loadercomparev410v420-2019-08-07) [Compare Source](https://togithub.com/webpack-contrib/file-loader/compare/v4.0.0...v4.1.0) ##### Features - `postTransformPublicPath` option ([#​334](https://togithub.com/webpack-contrib/file-loader/issues/334)) ([c136f44](https://togithub.com/webpack-contrib/file-loader/commit/c136f44)) ### [`v4.0.0`](https://togithub.com/webpack-contrib/file-loader/blob/master/CHANGELOG.md#​410httpsgithubcomwebpack-contribfile-loadercomparev400v410-2019-07-18) [Compare Source](https://togithub.com/webpack-contrib/file-loader/compare/v3.0.1...v4.0.0) ##### Features - improved validation error messages ([#​339](https://togithub.com/webpack-contrib/file-loader/issues/339)) ([705eed4](https://togithub.com/webpack-contrib/file-loader/commit/705eed4))

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