Add file icons

Purpose

This adds our file_icons svgs as a icon sprite and as individual files to our npm package. We also add a process for updating the file icons and a preview.

We also ported: vue_shared/components/file_icon/file_icon_map.js.

Only icons for file types mentioned in file_icon_map.js are copied.

Checklist

For a new icon and/or illustration:

• If this adds an icon ensure that it follows the new icon checklists.
• If this adds an illustration ensure that it follows the new illustration checklists.