Adds `FileType` enum

[NatanFreeman]

This pull request replaces the constants DIRECTORY and FILE inside src-tauri/src/filesystem/mod.rs with an enum called FileType as suggested here #60. This is makes the code more idiomatic and improves efficiency as we are no longer storing an entire str in memory.

[phoenix-ru]

Your implementation doesn't actually differ from the string comparison in terms of efficiency, because Serde's Serialize/Deserialize uses strings, but not numbers. I suggest you follow the initial recommendation and add serde_repr

[NatanFreeman]

Your implementation doesn't actually differ from the string comparison in terms of efficiency, because Serde's Serialize/Deserialize uses strings, but not numbers. I suggest you follow the initial recommendation and add serde_repr

Thanks for the suggestion. The recent commit should implement it.