Update dependency broccoli-merge-trees to v4

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
broccoli-merge-trees	^3.0.0 -> ^4.0.0

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Release Notes

broccolijs/broccoli-merge-trees

### [`v4.2.0`](https://togithub.com/broccolijs/broccoli-merge-trees/blob/master/CHANGELOG.md#​420) [Compare Source](https://togithub.com/broccolijs/broccoli-merge-trees/compare/v4.1.0...v4.2.0) - Use new public location for volta-cli/action - switch to GH actions - bump all dependencies ### [`v4.1.0`](https://togithub.com/broccolijs/broccoli-merge-trees/blob/master/CHANGELOG.md#​410) [Compare Source](https://togithub.com/broccolijs/broccoli-merge-trees/compare/v4.0.0...v4.1.0) - [Enhancement] add `destDir` ### [`v4.0.0`](https://togithub.com/broccolijs/broccoli-merge-trees/blob/master/CHANGELOG.md#​400) [Compare Source](https://togithub.com/broccolijs/broccoli-merge-trees/compare/v3.0.2...v4.0.0) - [Breaking] Require Node 10 or >= 12 - Use Class Syntax - Upgrade dependencies

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Configuration

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