Add String#ensure_suffix and String#ensure_prefix

[MatheusRich]

Closes #15783

This method returns self if the string ends with the given suffix. Otherwise, returns a new string with the suffix appended.

I've seen this pattern repeated several times along the years. Crystal itself does it in at least two places:

crystal/src/compiler/crystal/crystal_path.cr

Line 132 in 9791f31

yield relative_filename.ends_with?(".cr") ? relative_filename : "#{relative_filename}.cr"

crystal/src/crystal/system/wasi/wasi.cr

Line 27 in 9791f31

preopens << {path, path.ends_with?("/") ? path : path + "/", fd}

I'm open to other names, if ensure_suffix isn't clear enough. If this is merged and the core team agrees, I can add ensure_prefix as well.

[straight-shoota]

I like this, it seems useful.
Please open an issue first to further discuss the feature independent of its implementation (c. f. https://github.com/crystal-lang/crystal/blob/master/CONTRIBUTING.md#using-the-issue-tracker). Thanks 🙇

[MatheusRich]

🤦🏽 Sorry. Will do! @straight-shoota

[MatheusRich]

Done in #15783

[straight-shoota]

We could already use these new methods in the locations mentioned in the OP.

[MatheusRich]

@straight-shoota do we want to do that as part of this PR?

[straight-shoota]

Yes, I think this would fit here. Could be a follow-up as well.

[MatheusRich]

@straight-shoota I'd love to do it as follow-up, then (Draft PR here).