[6.x] Add TKeyedArray::make() factory to 6.x for cross-version plugin compatibility

[alies-dev]

Summary

Backports the TKeyedArray::make() static factory method from master to 6.x, so plugin authors can use the same API (TKeyedArray::make($properties)) on both Psalm 6.x and master without version-checking.

Motivation

Currently, plugin code that constructs a TKeyedArray must branch on Psalm version:

• 6.x: new TKeyedArray($properties) works (public constructor), but make() doesn't exist.
• master: TKeyedArray::make($properties) is the only way (private constructor).

With this change, TKeyedArray::make($properties) works consistently on both branches.

Design decisions

• new self() not new static() — avoids a silent argument-count mismatch when the method is inherited by TCallableKeyedArray, which has a 4-parameter constructor (no $is_list). Using static would mis-bind the 4th arg ($is_list) into $from_docblock.
• Return type self — master's make() can return TArray for the never-type edge case; this 6.x shim delegates to the constructor which always returns static. Returning self is correct and honest.
• @psalm-api on class — matching master; prevents Psalm dead-code detection from flagging make() as unused if no internal call sites exist.
• Public constructor unchanged — full backward compatibility for existing new TKeyedArray() callers.

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Note

Low Risk
Additive API-only change: introduces a static factory and @psalm-api annotation without altering existing constructor behavior. Low risk aside from potential downstream reliance on the new method signature.

Overview
Adds a new TKeyedArray::make() static factory (marked @psalm-pure) that forwards to the existing constructor, enabling a consistent construction API for plugins across Psalm versions.

Marks TKeyedArray as @psalm-api to formalize the public surface and avoid it being treated as internal/unused.

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