Cherry pick #1107 Migrate in-memory file system to new backend

[Jay Bosamiya (Microsoft) (jaybosamiya-ms)]

Note that this cherry-pick onto ulitebox is not clean, because ulitebox makes assumptions (due to #894) on how permission checks on directories should behave. Specifically, the Windows shim forces an incorrect implementation within the core, that would force incorrect behavior at the Linux shim. A proper fix would need a broader set of changes, so will need to be done separately. For now, this is documented by a comment and any time this is triggered at runtime, a debug log is printed.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/struct_missing.ron

Failed in:
  struct litebox::fs::in_mem::FileSystem, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/cfd1a2cacc07e2bf87112669ea51081b3de59328/litebox/src/fs/in_mem.rs:35