feat(@toss/utils): flatten array

[Ubinquitous]

Overview

A function that flattens a two-dimensional array into a one-dimensional array.

PR Checklist

• [x] I read and included theses actions below

1. I have read the Contributing Guide
2. I have written documents and tests, if needed.

[vercel[bot]]

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[BasixKOR]

Hello, we no longer maintain @toss/utils package and we recommend es-toolkit for all users. Please use flatten from es-toolkit instead!