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)

[Hi-Angel]

Please, don't fix older commits in newer ones

A problem introduced by commit X should be fixed in commit X, otherwise reviewing the code becomes very hard, as well as reading the git history after this gets merged.

See also this post from Linux HID subsystem and libinput maintainer Peter Hutterer. Despite the post age it's well relevant today, "old but gold".

[triallax]

@Hi-Angel thanks for the advice, but i'm not interested in purescript anymore (i haven't used it in years), feel free to take this up if you'd like