feat(stack): stack save/amend use $EDITOR

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Description

• Problem: git stack save/amend prompts for git commit messages only allow a single line commit.
• Solution: allow users to open commit messages in their editor.

In the case of glab stack amend use the previous commit as the initial value, allow people to use Esc to accept the default.

Related Issues

Resolves #7606 (closed)

How has this been tested?

• Manually tested
  • Go 1.22.5
  • Git 2.39.2
  • Debian bookworm, urxvt, env: EDITOR=vim

Screenshots (if appropriate):

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