Renames trashcan-16 icon to trash-16

[fermion]

Fixes #537

I've been using primer for a personal project of mine and noticed last night that one of the trash and trashcan icons was incorrectly named.

🙇🏻

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[colebemis]

Thanks for the PR, @fermion! ❤️

This is technically a breaking change so I'll handle releasing a new major version.