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!

Beginne damit, die MIT LICENSE-Dateien in CodeEditSourceEditor und CodeEditTextView mit dem Repository-Root von CodeEditSymbols zu vergleichen. Bestätige mit einem Maintainer, ob MIT vorgesehen ist, und füge dann die entsprechende LICENSE-Datei im Root hinzu; das Issue ist abgeschlossen, wenn das Paket seine Lizenz konsistent mit den referenzierten Schwesterpaketen deklariert.

Vom Indexierungsmodell aus dem Issue-Text verfasst.