Add upm to the package manager selector #3295

Describe the feature

The package page offers commands for several package managers, but upm is missing.

Add upm to the selector with upm add for installs, upx for package binaries, and upm create for create packages. Use the u from the upm wordmark as its icon.

A working implementation is available on this branch.

Additional information

• Would you be willing to help implement this feature?

Final checks

• Read the contribution guide.
• Check existing issues.