Rename USE_EMOJI to PIPX_USE_EMOJI

[dkav]

How would this feature be useful?
It would make easily discernible that this environmental variable is used by pipx.

Describe the solution you'd like
Replace USE_EMOJI with PIPX_USE_EMOJI

Describe alternatives you've considered
PIPX_NO_EMOJI (which is similar to Homebrew's HOMEBREW_NO_EMOJI)

[huxuan]

The suggestion is good, but I have a little concern about back-compatibility. Maybe we should support the PIPX_USE_EMOJI first and have a deprecation warning for USE_EMJOI usage?

[dkav]

That sounds fair to me.

[uranusjr]

We need to be careful with emitting a deprecation warning since the environment varialbe may have been set for other reasons. Maybe we should just keep supporting USE_EMOJI indefinitely but only recommend and document the variable prefixed with PIPX_.