🚀 Rewrite icon package

[mitchelloharawild]

This is a complete rewrite of the package to improve maintainability and performance. This chance will also move the repository from ropenscilabs/icon to its new home at mitchelloharawild/icon.

New icon sets

• Added the simple_icons() icon set (https://github.com/simple-icons/simple-icons/).
• Added the material_design() icon set (https://github.com/google/material-design-icons/).

Breaking changes

• Icons are no longer included directly in the package, and will require downloading before use.
• Animations of icons is no longer supported (this functionality will be re-introduced as a new package).
• The iconset_icon interface for accessing the icon icon from the iconset icon set has been replaced with iconset$icon.
• The iconset() interface for accessing the icons is now specific to each icon library.
• The short names (fa, ii, etc.) have been replaced with longer, more informative names (fontawesome, ionicons, etc.).

Improvements

• Icons now use SVG files instead of font files. This substantially reduces the output file's size and allows more flexibility and integrations with other libraries in the future.
• Icon sets can now be updated at any time without updating the package. The download_*() helpers can be used to install icon libraries.
• Custom icon sets can be created from a folder of SVG files.
• Icons can now be used with word_document and github_document output formats.
• Improved detection of output format types to work with more rmarkdown extensions.

Bug fixes

• Icons now work as expected in titles.
• Icons now work alongside emojis.