Update dependency webpack-merge to v5.1.4

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
webpack-merge	devDependencies	patch	5.1.1 -> 5.1.4

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Release Notes

survivejs/webpack-merge

### [`v5.1.4`](https://togithub.com/survivejs/webpack-merge/blob/master/CHANGELOG.md#​514--2020-09-09) [Compare Source](https://togithub.com/survivejs/webpack-merge/compare/v5.1.3...v5.1.4) - Fix - Expose `CustomizeRule` for TypeScript users [#​147](https://togithub.com/survivejs/webpack-merge/issues/147) ### [`v5.1.3`](https://togithub.com/survivejs/webpack-merge/blob/master/CHANGELOG.md#​513--2020-08-30) [Compare Source](https://togithub.com/survivejs/webpack-merge/compare/v5.1.2...v5.1.3) - Fix - Allow merging a `RegExp` to itself [#​145](https://togithub.com/survivejs/webpack-merge/issues/145) ### [`v5.1.2`](https://togithub.com/survivejs/webpack-merge/blob/master/CHANGELOG.md#​512--2020-08-18) [Compare Source](https://togithub.com/survivejs/webpack-merge/compare/v5.1.1...v5.1.2) - Fix - Allow overriding an object field with `null` [#​144](https://togithub.com/survivejs/webpack-merge/issues/144)

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