rename data_dir to data

[Ocraftyone]

Description

Sets the default data_dir to just data

[SoftFever]

Thanks @Ocraftyone

I understand where you come from.
I chose to use data_dir instead of just data to avoid potential naming conflicts.
While this is unlikely to be an issue for Windows, where we'll place the data folder alongside the executable file within the specific Orca Slicer directory, the situation differs for Mac, as it's a bundled.

So I think it's better to use a slightly longer name is a safe choice here

e.g.
Some folder:
- OrcaSlicer.app
- xxx.app
- xxx(folder)
- data_dir

[Ocraftyone]

@SoftFever How about orca_data instead? That would alleviate the concern for conflict and I personally think it is a bit of a better naming choice.