Very nice to see a fellow godot dev :)
You can tick the "Flip H" box in the property-explorer on the right, when setting up your animation keyframes with the animation player or just with code via something like this:
if current_direction == "left":
$Sprite.flip_h = true
$AnimationPlayer.play("Walk")
Here is a tutorial by DevWorm where he does it with an AnimatedSprite: https://www.youtube.com/watch?v=pBoXqW4RykE
The flip happens at 15:50
I hope this helps, have fun with your project!