[Feature] file extension and file icon for blocks projects

[laurensvalk]

Blocks should have a different file extension and icon to make them easily identifiable compared to python scripts.

Originally posted by @dlech in https://github.com/pybricks/pybricks-blocks/issues/25

[laurensvalk]

I think the extension can stay since they are valid Python scripts, but a different icon would be very helpful.

Sorting them separately in the file browser might also be nice.