Add a LICENSE file (the package currently has none)

Hi, and thanks for the package — the editor stack is a pleasure to build on.

This repository has no license file. There is none in the repository root, none
in the source tree, and GitHub's API reports `"license": null` for it. The same
is true of the sibling package (CodeEditLanguages), while
CodeEditSourceEditor and CodeEditTextView both carry MIT.

Why this matters downstream: without a license the default applies, and the
default is "all rights reserved" — no permission to redistribute. Both packages
are listed in the `CodeEditSourceEditor` target's own dependencies, so every
application that uses the editor redistributes them inside its binary. I ship a
macOS application built on CodeEditSourceEditor, and when I put together the
attribution notice for it I could only write "no license declared upstream" for
these two, which is not a comfortable line to publish and is presumably not what
you intended either.

If MIT was the intent — as it is for the rest of the project — adding the usual
`LICENSE` file would settle it. If a different license is intended, that would be
just as useful to know; the point is only that the file be there.

Happy to open a PR with an MIT `LICENSE` matching CodeEditSourceEditor's if that
helps.

Thanks!

Commencez par comparer les fichiers MIT LICENSE de CodeEditSourceEditor et CodeEditTextView avec la racine du dépôt de CodeEditSymbols. Confirmez avec un maintainer que MIT est bien prévu, puis ajoutez le fichier LICENSE approprié à la racine ; l’issue est terminée lorsque le package déclare sa licence de manière cohérente avec les packages frères référencés.

Rédigé par le modèle d'indexation à partir du texte de l'issue.