Some of the files in the Arduino core were missing the license headers.

They contain code which is either copied from previous Arduino cores
(which were already LGPL-2.1-or-later), or has been submitted to this
repository where most of the code is clearly LGPL-2.1-or-later.  In any
case, all the contributors have either been directly employed by Arduino
or have signed the CLA.

Add the missing LGPL-2.1-or-later headers to the files.

The CAN files had a non-standard license which does not match the other
files in the repository. This commit updates the license header to
match that of the other files.

All of the contributors to these files have either been employed by
Arduino or have signed the CLA.

Minor updates to the license headers to make them more consistent among
the repository.

The files in the test/ folder had a copyright notice, but no license
information. This commit adds the LGPL-2.1-or-later SPDX license
identifier to all these files.

All files in this repository are licensed under the GNU Lesser General
Public License version 2.1. Make this obvious by including the LICENSE
file in the root of the repository.

In addition to the above clause, some files may also be distributed under
additional license terms. See each individual file for details.

The codespell spellchecker tool is used to automatically detect commonly misspelled words in the files of this project.

The misspelled words dictionary was expanded in the latest release of codespell. Some of the text in the project
codebase happens to match against newly added entries, which caused codespell to produce a false misspelled word
detection.

Since the code that produced the detection is correct and intended, the false positive is resolved by configuring
codespell to ignore the problematic word.

Fix spell check false positive by ignoring word

Use the LGPL 2.1 or later license on all files