Add a Join() function to the lo package

[eliotttak]

Describe your changes

I added a Join() function to the lo package.
It takes a collection and a separator (that can be any type) and returns a string that contains
every item, separated by the specified separator.

It uses fmt to convert list items and separator to string, and a strings.Builder to construct it.
I created an example on pkg.go.dev but because the import path is github.com/samber/lo, it will be available only if you accept the PR.

I wrote various tests in ./slice_test.go. Feel free to look at them if you want to verify, I tried to cover every case where my function could bug.

Checklist before requesting a review

• 👓 I have performed a self-review of my code
• 👶 This helper does not already exist
• 🧪 This helper is tested
• 🏎️ My code limits memory allocation and is fast
• [?] 🧞‍♂️ This helper is immutable and my tests prove it
• [?] ✍️ I implemented the parallel, iterator and mutable variants
• 🔬 An example has been added to lo_example_test.go
• ⛹️ An example has been created on https://go.dev/play and added in comments
• 📖 My helper has been added to documentation
  • in README.md
  • in docs/data/*.md
  • in docs/static/llms.txt

Conventions

• Returning (ok bool) is often better than (err error)
• panic(...) must be limited
• Helpers should receive variadic arguments when relevent
• Add variants of your helper when relevant:
  • Variable number of arguments: lo.Must0, lo.Must1, lo.Must2, ...
  • Predicate with index: lo.SliceToMap vs lo.SliceToMapI
  • With lazy callback: lo.Ternary vs lo.TernaryF
  • ...

[eliotttak]

Fixes #351.