Add/licensing

[jeffpaul]

Description of the Change

This PR adds the LICENSE.md markdown file for GPLv2 so that GitHub will recognize and display the licensing, adds the relevant license identifier on necessary files across the repo, and adds in missing header fields on various files.

One thing that I noticed was missing in wpengine-phpcompat.php and readme.txt was the Requires PHP header value, so confirming that that should be set to and adding that in would be ideal on top of this PR.

Note that this also adds the LICENSE.md file to .distignore though the .distignore file has some reorganization as part of #282, so assuming 282 is merged in first we'll need a slight refresh to ensure that the license file is properly excluded from distributed/released versions of the plugin.

Alternate Designs

none identified, ideally the repo should be explicit with its licensing.

Benefits

Ensures that people looking to understand how this repo is licensed do not need to go digging into various files to see what header fields state the licensing is.

Possible Drawbacks

None identified, so long as GPLv2-or-later is agreed as the proper licensing.

Verification Process

Manually verified via VS Code and GitHub Desktop UI.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

none identified

Changelog Entry

### Added
- Plugin license file and various file license headers for `GPL-v2-or-later` (props [@jeffpaul](https://github.com/jeffpaul/)).