Make reflect work for ElementInternals

Other changes:

* Remove reflection of unrestricted double as it is buggy and unused.
* The DOMString getter steps did not account for a missing attribute.
* The native accessibility semantics map was renamed to the internal content attribute map as it's now a more general reflection concept.

Corresponding ARIA PR: w3c/aria#1876.

Fixes #8442.

Follow-up:

* w3c/core-aam#152
* w3c/aria#1110
* #3238
* #8544
* #8545
* #8926
* #8927
* #8928
* #8930

Author: annevk