Introducing template-tag feature flag

[ef4]

This is a working implementation of feature flagging, and an initial bit of content using the template-tag feature flag.

The only affordance to activate a default-disabled flag is to edit it into a query param.

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[kategengler]

PR to fix Trailing History empress/guidemaker#117

[ef4]

PR is updated to assume the guidemaker fix has landed (which it has not right now). It should be green if that published as a compatible release.

[ef4]

There is a sporadic CI failure. I've confirmed that when it happens its a fetch failure:

TypeError: Failed to fetch
    at ApplicationAdapter._fetchRequest (http://localhost:7357/assets/vendor.js:22662:16)
    at ApplicationAdapter._callee$ (http://localhost:7357/assets/vendor.js:22605:29)
    at tryCatch (http://localhost:7357/assets/vendor.js:21627:1064)
    at Generator.<anonymous> (http://localhost:7357/assets/vendor.js:21627:3010)
    at Generator.next (http://localhost:7357/assets/vendor.js:21627:1701)
    at asyncGeneratorStep (http://localhost:7357/assets/vendor.js:21628:72)
    at _next (http://localhost:7357/assets/vendor.js:21629:165)
    at http://localhost:7357/assets/vendor.js:21629:301
    at new Promise (<anonymous>)
    at ApplicationAdapter.<anonymous> (http://localhost:7357/assets/vendor.js:21629:92)"}

[kategengler]

https://github.com/ember-learn/guides-source/actions/runs/12379836293/job/34554877026 on a merge of a content change failed the same way -- lending suspicion of the ember + ember-data upgrade (that moved to real fetch).