Create dependabot.yml

[IsakTheHacker]

Issue description

I saw that some dependencies in the GitHub action workflows were out of date. Since GitHub is always migrating to newer Node runtimes it is important to keep GitHub action dependencies up to date so they won't become incompatible all of a sudden.

Solution description

I've added a dependabot configuration that will check for updates to GitHub actions used in the workflow files weekly.

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[rdb]

Which dependencies does this affect, exactly?

[IsakTheHacker]

It affects GitHub action dependencies in workflow files located in the .github/workflows directory