Update dependency @webcomponents/custom-elements to v1.6.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@webcomponents/custom-elements (source)	1.2.0 -> 1.6.0

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Release Notes

webcomponents/polyfills (@​webcomponents/custom-elements)

v1.6.0

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• Add support for Element.toggleAttribute() (#​541)
• Update dependencies (#​542)

v1.5.1

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• Don't write to window.CustomElementsRegistry unless we're also writing to window.customElements. (#​524)
• Update TypeScript and upstream internal changes.
  (#​525)

v1.5.0

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• Add TS externs. (#​457)

v1.4.3

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• Maintenance release (no user-facing changes)

v1.4.2

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• Type annotation improvements for Closure (#​287)
• README improvements (#​169)

v1.4.1

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• Fix bug that prevented elements in the main document from being upgraded
  (#​283)

v1.4.0

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• Convert to TypeScript
  (#​246)

v1.3.2

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• Maintenance release (no user-facing changes)

v1.3.1

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• Remove warnings about insertAdjacentElement and insertAdjacentHTML not
  being patched (#​229)
• Fix incorrect polyfillWrapFlushCallback example in README
  (#​219)

v1.2.1

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