Add str append to stdlib-candidate

[savente93]

As discussed in nushell/nushell#10486 I've added an stdlib-candidate folder where we can add scripts that might want to be in std-lib at some point. Currently it only contains str append and str prepend which work about how you'd expect. Thanks to @amtoine for writing the initial function. I added a default branch that just returns the input unaltered so it can be used more easily in the middle of a pipe.

[fdncred]

This is great! Thanks! Would you mind putting a readme.md in the folder describing what this folder is for? Like what you said above? Just so, when other people look into this folder, they have some idea of what is going on.

[savente93]

good one! one sec.

[fdncred]

Nice words in the readme. Thanks!

[amtoine]

i like this idea 👌

@savente93
could you add tests for these two commands as well? that'd be perfect 😇

[savente93]

sure thing, I'll try and open a PR somwhere today if I have time!

[amtoine]

amazing 🙏

just some

#[test]
def test_1 [] {
    ...
}

next to the definitions should be enough 😉