Update Suffix.swift

[mattyoung]

Change endOfPrefix(while:) and startOfSuffix(while:) from internal to public

These two functions are generally useful so they should be public instead of internal
See https://forums.swift.org/t/should-there-be-bidirectionalcollection-droplast-while/48059/14
and https://forums.swift.org/t/endofprefix-while-and-startofsuffix-while-should-be-public-instead-of-internal/56855

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[lucianopa-msft]

Since we are making those public, couple of points are think are worth mention:

• We probably need to add this to Changelog.md
• We also may need a Guide the same way we have for all other algorithms here. See Guides for reference.
• Add entry to the Readme.md
• Verify if we have tests for those methods? I believe we test that indirectly through the public methods that use them internally, but not sure we have tests for them specifically (probably have, but maybe it is worth check)

[natecook1000]

Landing this change in #211.