[WebAPI] Implement Declarative Shadow DOM

[Rumyra]

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
• Add entry to Firefox experimental features page if feature is not yet enabled in release

Related Gecko bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1712140

Related PRs

• Add shadowrootmode in Firefox preview browser-compat-data#21815
• Update template element description and add shadowrootmode attribute #31490
• Add a declarative shadow DOM example #31577
• Add experimental features for Fx 122: shadowrootmode, async clipboard #31612
• Add shadowrootmode examples interactive-examples#2707
• Add Shadow DOM examples dom-examples#245

Related issues

• The shadowrootmode attribute on <template> is about to be standardized #29600
• Add information to DOMParser.parseFromString page about the includeShadowRoots option #31501

Other

• Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.
• Check if glossary updates are required for the feature you're documenting - whether an existing term needs to be updated or a new term should be added.
• Check if BCD update means that content pages need to have experimental markup removed or deprecated markup added (front matter tags and macros).

[pepelsbey]

⚠️ it seems like a Nightly-only feature, based on the feedback I got in the BCD PR.