Allow to overwrite the $choices-z-index variable

[johannlejeune]

Description

#794 introduced a new SCSS variable for configuring the z-index, but it didn't set the !default flag, making it impossible to overwrite in external files.

Types of changes

• Chore (tooling change or documentation change)
• Refactor (non-breaking change which maintains existing functionality)
• 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 not work as expected)

Checklist

• My code follows the code style of this project.
• I have added new tests for the bug I fixed/the new feature I added.
• I have modified existing tests for the bug I fixed/the new feature I added.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[aaronjchristian]

Yep this looks good to me!