first draft for ux icons support

[kevinpapst]

Description

This is a first draft for UX icons integration. It supports both Fontawesome webfont and UX icons.

It ships a command that can convert current webfont configuration and download all icons configured at tabler.icons, and it even supports converting old FA5 to FA6 names (the webfont supported both versions, iconify only the new names).

The tabler_icon() Twig functions works as before and supports both implementations. The |tabler_icon Twig filter will still work with webfonts, but not with UX icons.

The biggest issues I see:

• It increases the page size by 20kB for each request
• And many icons cannot be cached e.g. the entire menu is based on runtime events and those icons won't be cached (according to the docs). That means for every page load PHP will read all these SVGs from disk? If that is true (I need to investigate further) this is a blocker for me as of now.

Closes #194

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [ ] I updated the documentation (see here)
• [ ] I agree that this code will be published under the MIT license

[cavasinf]

Big 👍 for splitting RuntimeExtension in IconExtension.