fix(overlay): handle missing customElements

[daaku]

Description

My environment doesn't have customElements. I set server.hmr.overlay to false, but importing overlay immediately triggers this error, making the configuration moot for this case.

Additional context

In theory a better fix might be to use an async import for overlay. async imports potentially have other issues depending on the build/env so I avoided using that approach.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[daaku]

Sweet - that was fast.