Don't export Collection subscript(safe:) extension

[boctor]

Description

This PR fixes a minor bug where the Collection subscript(safe:) extension was exposed outside of the library causing the error ambiguous use of 'subscript(safe:)' in host apps that have defined the same extension.

Change Notes

• Removed public from the Collection subscript(safe:) extension to keep it private to the mamba library

Pre-submission Checklist

• [X] I ran the unit tests locally before checking in.
• [X] I made sure there were no compiler warnings before checking in.
• [ ] I have written useful documentation for all public code.
• [ ] I have written unit tests for this new feature.

[CLAassistant]

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