Add Size::dimensions() for destructuring (fixes #1480)

[Abdooo2235]

Summary

Closes #1480.

Adds Size::dimensions(): array{0: int, 1: int} so consumers can destructure
size into a [width, height] int pair:

[$width, $height] = $image->size()->dimensions();

Why not change [$w, $h] = $image->size() directly?

The literal syntax in the original issue would require changing
ArrayAccess::offsetGet on Size. That offset is currently the way
Polygon (Size's parent) exposes the four corner Point objects, and
Size::setWidth()/setHeight() rely on it internally
(\$this[1]->setX(...), etc.). Repurposing those offsets to return ints
would be a breaking change for every caller and for the class's own setters.

dimensions() is the non-breaking alternative: same ergonomics for
destructuring, zero risk for existing code.

Tests

Added to tests/Unit/SizeTest.php:

• testDimensionsReturnsWidthAndHeightAsInts
• testDimensionsEnablesArrayDestructuring
• testDimensionsReflectsSetWidthAndSetHeight
• testArrayAccessStillReturnsCornerPoints (explicit BC guard)

vendor/bin/phpunit tests/Unit/SizeTest.php passes locally (67 tests, 152 assertions).

Notes

• SizeInterface updated to include the new method.
• No changes to Polygon or ArrayAccess semantics.