feat: add descriptions for bash and powershell autocompletion

[gabelluardo]

What type of PR is this?

• bug
• feature

What this PR does / why we need it:

This PR fixes a long standing issue with subshells, adding by default commands descriptions to all the supported shells.

• bash_autocomplete and powershell_autocomplete added the ability to handle command descriptions
• example_test.go fixed output for the new behaviour
• help.go removed the environment variable dependency

Which issue(s) this PR fixes:

Fixes #1884

Special notes for your reviewer:

I'm a noob in bash so I think there is a big margin to improve that code.
It works, but suggestions are really welcome.

Testing

I used my own projects to test it with bash, zsh and fish, searching regressions.

I'm not able to test the powershell implementation, but ideally should work

Release Notes

feat: add descriptions for bash and powershell autocompletion