Update return type for `columns` function prop to `number | undefined`

[Aleksandr Hovhannisyan]

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.