AK: Add factory methods for creating smart pointers

These functions abstract away the need to call the proper new operator
("throwing" or "non-throwing") and manually adopt the resulting raw
pointer. Modelled after the existing `NonnullOwnPtr<T> make()`
functions, these forward their parameters to the object's constructor.

Note: These can't be used in the common "factory method" idiom, as
private constructors can't be called from a standalone function.

The naming is consistent with AK's and Shell's previous implementation
of these:
- `make` creates a `NonnullOwnPtr<T>` and aborts if the allocation could
  not be performed.
- `try_make` creates an `OwnPtr<T>`, which may be null if the allocation
  failed.
- `create` creates a `NonnullRefPtr<T>`, and aborts on allocation
  failure.
- `try_create` creates a `RefPtr<T>`, which may be null if the
  allocation was not successful.

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