Update dependency file-loader to v4

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
file-loader	devDependencies	major	2.0.0 -> 4.0.0

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Release Notes

webpack-contrib/file-loader

### [`v4.0.0`](https://togithub.com/webpack-contrib/file-loader/blob/master/CHANGELOG.md#​400httpsgithubcomwebpack-contribfile-loadercomparev301v400-2019-06-05) [Compare Source](https://togithub.com/webpack-contrib/file-loader/compare/v3.0.1...v4.0.0) ##### chore - **deps:** update ([#​333](https://togithub.com/webpack-contrib/file-loader/issues/333)) ([0d2f9b8](https://togithub.com/webpack-contrib/file-loader/commit/0d2f9b8)) ##### BREAKING CHANGES - **deps:** minimum required nodejs version is `8.9.0` ### [`v3.0.1`](https://togithub.com/webpack-contrib/file-loader/blob/master/CHANGELOG.md#​400httpsgithubcomwebpack-contribfile-loadercomparev301v400-2019-06-05) [Compare Source](https://togithub.com/webpack-contrib/file-loader/compare/v3.0.0...v3.0.1) ##### chore - **deps:** update ([#​333](https://togithub.com/webpack-contrib/file-loader/issues/333)) ([0d2f9b8](https://togithub.com/webpack-contrib/file-loader/commit/0d2f9b8)) ##### BREAKING CHANGES - **deps:** minimum required nodejs version is `8.9.0` ### [`v3.0.0`](https://togithub.com/webpack-contrib/file-loader/blob/master/CHANGELOG.md#​300httpsgithubcomwebpack-contribfile-loadercomparev200v300-2018-12-20) [Compare Source](https://togithub.com/webpack-contrib/file-loader/compare/v2.0.0...v3.0.0) ##### Code Refactoring - drop support for webpack < 4 ([#​303](https://togithub.com/webpack-contrib/file-loader/issues/303)) ([203a4ee](https://togithub.com/webpack-contrib/file-loader/commit/203a4ee)) - more validations in `options` schema ##### Features - `resourcePath` is now available in `outputPath` and `publicPath` ([#​304](https://togithub.com/webpack-contrib/file-loader/issues/304)) ([0d66e64](https://togithub.com/webpack-contrib/file-loader/commit/0d66e64)) - `context` is now available in `outputPath` and `publicPath` ([#​305](https://togithub.com/webpack-contrib/file-loader/issues/305)) ([d5eb823](https://togithub.com/webpack-contrib/file-loader/commit/d5eb823)) ##### BREAKING CHANGES - removed the `useRelativePath` option. It is dangerously and break url when you use multiple entry points. - drop support for webpack < 4

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