Add GitHub Actions CI

[nikitalita]

This adds the following actions:

• Static checks for checking that files respect clang-format
• Building the editor with gdsdecomp on Linux and Windows
• Building the export template, then downloading the editor from the previous job and exporting the standalone project
• A scheduled build (every 6 hours) against the current master to see if and when gdsdecomp has drifted out of sync with the API.

Because godot master still doesn't support --no-window, swiftshader is used to start godot on the headless action runners.

This also fixes a number of warnings that were in the code, like a warning raised when using an unsigned int with index check.

[nikitalita]

Synced with the latest changes on the Godot master API; this was caught by the CI.

[nikitalita]

Is there anything else you want me to do on this PR?

[bruvzg]

Thanks!