Update JavaScript package to support 24px icons

[colebemis]

Problem

As part of the Octicons refresh project, we designed a set of 24px icons—as well as 16px icons—to accommodate interfaces that need larger icons. With the addition of this 24px grid size, we no longer have a 1-to-1 mapping of icon name to SVG file.

Solution

The structure of each icon object has been updated to allow support multiple SVGs per icon:

Before

octicons.alert
// {
//     symbol: 'alert',
//     keywords: ['warning', 'triangle', 'exclamation', 'point'],
//     toSVG: [Function],
//     width: 16,
//     height: 16,
//     path: '<path d="M8.865 1.52c-.18-.31-.51-.5-.87-.5s-.69.19-.87.5L.275 13.5c-.18.31-.18.69 0 1 .19.31.52.5.87.5h13.7c.36 0 .69-.19.86-.5.17-.31.18-.69.01-1L8.865 1.52zM8.995 13h-2v-2h2v2zm0-3h-2V6h2v4z"/>',
//     options: {
//         version: '1.1',
//         width: '16',
//         height: '16',
//         viewBox: '0 0 16 16',
//         class: 'octicon octicon-alert',
//         'aria-hidden': 'true'
//     },
// }

After

octicons.alert
// {
//     symbol: 'alert',
//     keywords: ['warning', 'triangle', 'exclamation', 'point'],
//     toSVG: [Function]
//     heights: {
//         16: {
//             width: 16,
//             path: '<path d="M8.865 1.52c-.18-.31-.51-.5-.87-.5s-.69.19-.87.5L.275 13.5c-.18.31-.18.69 0 1 .19.31.52.5.87.5h13.7c.36 0 .69-.19.86-.5.17-.31.18-.69.01-1L8.865 1.52zM8.995 13h-2v-2h2v2zm0-3h-2V6h2v4z"/>',
//             options: {
//                 version: '1.1',
//                 width: '16',
//                 height: '16',
//                 viewBox: '0 0 16 16',
//                 class: 'octicon octicon-alert',
//                 'aria-hidden': 'true'
//             },
//         },
//         24: ...
//     }
// }

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[jonrohan]

Changes look good, thanks for doing this 👍 and for the tests.