[ON_HOLD] feat: custom decorators

[runspired]

This allows Ember to more easily remove support for some aspects of ComputedProperty, computed.meta etc. while also giving us better access to the schema at runtime. This is the first step towards being able to compile schema at build time, some deprecations are necessary to take that step.

In theory, this should provide a significant performance boost for accessing static schema and/or accessing a value. However, do to the complexity of also supporting sync-observers and async-proxies there are some caveats that may negate these wins. Benchmarking is needed.

Edit Going to put this on hold for now because (1) the various workarounds for reopen and sync observers seem to nuke perf in addition to adding a lot more size and complexity and (2) there's no clear path forward for this issue: https://github.com/emberjs/ember.js/pull/19504