Update return type for columns function prop to number | undefined

[AleksandrHovhannisyan]

The way the code is written here (two if statements) suggests that the columns function prop can technically return undefined, allowing a user to fall back to the default internal calculations in the if statement on L82:

react-photo-gallery/src/Gallery.js

Lines 76 to 87 in 0bb8e4c

	if (direction === 'column') {
	// allow user to calculate columns from containerWidth
	if (typeof columns === 'function') {
	columns = columns(containerWidth);
	}
	// set default breakpoints if user doesn't specify columns prop
	if (columns === undefined) {
	columns = 1;
	if (containerWidth >= 500) columns = 2;
	if (containerWidth >= 900) columns = 3;
	if (containerWidth >= 1500) columns = 4;
	}

I was able to verify this locally. However, the return type for this function is strictly number, so I get type errors if I try to return undefined from the function:

react-photo-gallery/src/Gallery.js

Line 120 in 0bb8e4c

columns: PropTypes.oneOfType([PropTypes.func, PropTypes.number]),

I think it would be nice to update the prop types to allow returning undefined since it seems to work as expected: I can either specify custom logic or return undefined to signal that I want to fall back to the default calculations.

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Alternatively, the function could accept a second argument—the default number of columns for containerWidth—so you could do something like this:

columns={(containerWidth, defaultNumColumns) => containerWidth <= 320 ? 2 : defaultNumColumns}

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Happy to put in a PR for this if you approve.