Core: Cleanup Array/Dictionary implementation

[Repiteo]

• Partially supersedes Core: Migrate typed container logic #96741
• Partially supersedes Add std::initializer_list support to Array #89782
• Partially supersedes Add list initialization to Array, Variant, and TypedArray #86015

A collection of fixes/improvements for Arrays & Dictionaries, particularly the typed variants. In no particular order:

• Fixed std::initializer_list support on TypedDictionary by adding a few missing declarations & ensuring that Object types are passed as pointers.
• Added std::initializer_list support for Array/TypedArray along with dedicated tests.
• Moved Variant conversion logic from TypedArray/TypedDictionary to Variant via forward-declared templates.
• Consolidated typed_array.h specialization macros into one, much like typed_dictionary.h.
• Migrated ArrayPrivate/DictionaryPrivate; they're now nested within their respective base classes.