Create dependabot.yml for action updates

[XhmikosR]

Should be merged after #491

EDIT: let me know if you want me to add cargo too.

[shssoichiro]

Thanks, this looks good to me. As for Cargo dependencies, I usually update them in bulk before publishing a release. In the past Dependabot has frustrated me with dependency updates because it creates a separate PR for each individual update, which gets extremely noisy/tedious at times. If there was a way to weekly batch all patch updates as one PR, that would make it more palatable. Dependabot, last I checked, lacks this functionality. We attempted to do this in rav1e using an alternate tool (renovate) and it seems to fail to pick up a majority of the dependencies, so that was also disappointing.