Add LeanCode and contributors to AUTHORS (#11134)

This PR adds [LeanCode](https://leancode.co) (a Flutter development company) and three of its contributors to the AUTHORS file.

Contributors added:
- **LeanCode** — company
- **Piotr Denert** (GitHub: [pdenert](https://github.com/pdenert)) — PRs in flutter/packages
- **Marcin Chudy** (GitHub: [mchudy](https://github.com/mchudy)) — PRs in flutter/packages
- **Paweł Jakubowski** (GitHub: [FirentisTFW](https://github.com/FirentisTFW)) —  PRs in flutter/packages

No issue is required as this is a trivial AUTHORS file update.

## Pre-Review Checklist

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AUTHORS

@@ -82,3 +82,7 @@ Ricardo Dalarme <ricardodalarme@outlook.com>
 Andrei Kabylin <sys.int64@gmail.com>
 Ernesto Ramirez <neartox@outlook.com>
 Rui Craveiro <ruicraveiro@squarealfa.com>
+LeanCode <contribution@leancode.pl>
+Piotr Denert <pdenert09@gmail.com>
+Marcin Chudy <marcinchudy94@gmail.com>
+Paweł Jakubowski <pawel.jakubowski@leancode.pl>