feat(filesystem): Introduce fatfs as lib

[Koichi98]

Description

In a previous pull request, FatFs was introduced as a sub-crate directly under the project directory, similar to smoltcp. However, this pull request moves it into awkernel_lib.

When moving the FatFs crate, we're also elevating io.rs and error.rs to a higher level in the module hierarchy, above the FatFs module itself. This is a deliberate design choice, as it allows other file systems to leverage these shared traits and error types.

awkernel_lib/src/file$ tree
.
├── error.rs
├── fatfs
│   ├── boot_sector.rs
│   ├── dir_entry.rs
│   ├── dir.rs
│   ├── file.rs
│   ├── fs.rs
│   ├── LICENSE.txt
│   ├── table.rs
│   └── time.rs
├── fatfs.rs
├── io.rs

Related links

PR #444

How was this PR tested?

Notes for reviewers