fix: Prefix names starting with numbers with an underscore in slugify function

[Defiantearth]

Closes: #319

This PR builds on the original work from @johannwagner. Since the original PR was inactive, we created a new branch incorporating their changes and added the final updates needed to complete the fix.
You can find the original pull request here.

What's Changed

Fixed an incompatibility when slugifying capirca names that start with a number by prefixing the name with an underscore.

To Do

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• Attached Screenshots, Payload Example
• Unit, Integration Tests
• Documentation Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
nautobot_firewall_models/utils
capirca.py
Project Total

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[gsnider2195]

Looks good!