Add GitHub workflows

[Dillon-Brown]

Description

• Added composer validation workflow (composer.json, composer.lock, codecov.yml).
• Added code validation workflow (PHPLint).
• Added Security check workflow (DevSkim, Symfony).
• Renamed .codecov.yml to codecov.yml (Appears to be the standard).
• Removed the Travis composer self-update (Stable is a default).
• Removed the Travis PHPLinter (Now redundant).
• Removed the Travis composer validate job (Now redundant).

How To Test This

1. See PR checks.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Final checklist

• My code follows the code style of this project found here.
• My change requires a change to the documentation.
• I have read the How to Contribute guidelines.

[samus-aran]

Hey @Dillon-Brown Hows best to assess this? On my own branch?

[Dillon-Brown]

@samus-aran You can see the changes directly here under the "checks" tab. Otherwise if you want to test it on your own repo you should be able to pull across from this branch, you'd just need to enable GitHub actions in project settings.