feat(variables): add option to list/update/set description of vars

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Description

• feat(variables): add option to list/update/set description of vars.

I also expanded the tests a bit.

Related Issues

Resolves #7738 (closed)

Screenshots (if appropriate):

> glab var list
Listing variables for the rndmh3ro/example project:

KEY        PROTECTED  MASKED  EXPANDED  SCOPE
foo        false      false   true      *
fooz       false      false   true      *
with_desc  false      false   true      *


> bin/glab var list
Listing variables for the rndmh3ro/example project:

KEY        PROTECTED  MASKED  EXPANDED  SCOPE  DESCRIPTION
foo        false      false   true      *
fooz       false      false   true      *
with_desc  false      false   true      *      bar

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)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap