Add startsWith and endsWith

[triallax]

Description of the change

Add startsWith and endsWith functions to Data.String.CodeUnits.

Fixes #127.

I know that there's still a bit of disagreement over whether this should be done or not, but I just opened this PR because it didn't take a lot of time. If it's ultimately decided against, feel free to close this PR.

To do:

• [x] suggest stripPrefix and stripSuffix
• [x] don't use FFI functions, instead use stripPrefix and stripSuffix
• [x] add functions to Data.String.NonEmpty

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Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• [x] Linked any existing issues or proposals that this pull request should close
• [x] Updated or added relevant documentation
• [x] Added a test for the contribution (if applicable)