[Quest]: Proper ShadowDOM support

[NullVoxPopuli]

Goal:

• support rending in to a shadow-dom
• be able to render in to a shadow-dom in a single render pass (the current way to get around limitations in ember/glimmer to render in to a shadow requires an extra render pass as well as an extra couple elements)

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Currently, <template shadowrootmode="open"> is not allowed in template-tag:

• Add failing test for declarative shadow-dom embroider-build/content-tag#66

GlimmerVM doesn't render directly in to a shadow:

• [Bug] declarative ShadowDOM is unable to have content rendered in to it #20642
• [Bug] #in-element cannot directly render in to shadow-dom #20641
• [Bug] in-element cannot render into a document-fragment #20643
• Improve builtin shadow-dom workflow [add shadow node opcode] glimmerjs/glimmer-vm#1195 (closed due to being implemented in the GlimmerVM rewrite, Glimmer Next)

Related

• [Bug] Could not initialize ember app in shadow root node #19734

Regressions?

• action modifier doesn't fire when inside shadow root #18303 (resolved, but now no longer works, due to [Bug] #in-element cannot directly render in to shadow-dom #20641)