Update dependency webpack-merge to v5.1.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
webpack-merge	devDependencies	minor	5.0.9 -> 5.1.1

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Release Notes

survivejs/webpack-merge

### [`v5.1.1`](https://togithub.com/survivejs/webpack-merge/blob/master/CHANGELOG.md#​511--2020-08-04) [Compare Source](https://togithub.com/survivejs/webpack-merge/compare/v5.1.0...v5.1.1) - Fix - Drop `` from **index.d.ts** [#​143](https://togithub.com/survivejs/webpack-merge/issues/143) ### [`v5.1.0`](https://togithub.com/survivejs/webpack-merge/blob/master/CHANGELOG.md#​510--2020-08-03) [Compare Source](https://togithub.com/survivejs/webpack-merge/compare/v5.0.9...v5.1.0) - Feature - Expose `merge` as `default` for backwards-compatibility with TypeScript. Now `import merge from "webpack-merge";` works there. In CommonJS, the default alias is exposed through `default` property due to technical constraints.

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