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

Compare Source

• Use new public location for volta-cli/action
• switch to GH actions
• bump all dependencies

v4.1.0

Compare Source

• [Enhancement] add destDir

v4.0.0

Compare Source

• [Breaking] Require Node 10 or >= 12
• Use Class Syntax
• Upgrade dependencies

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