Implemented different with_native_path conversion methods, refactoring fs functions to utilize with_native_path, and move exists to use with_native_path

[asder8215]

This PR attempts to complete the FIXME comment on trying to introduce the different conversion functions of with_native_path on all supported platforms and also moves the exists filesystem functions to utilize with_native_path. It should be easier to make readdir and remove_dir_all utilize with_native_path, which I'm down to support that transition in a separate PR.

I took note on what each platform fs functions does underneath the hood to decide the argument it should take:

• On Hermit and Vexos they use run_path_with_cstr just like Unix, so it was clear for their fs functions to take a &CStr
• Motor always converts the &Path to &str, so I just changed the arguments to &str
• Solid uses an internal cstr function that converts a &Path to io::Result<CString>, so I made its fs functions take in a &CStr anyways
• On uefi, as far as I'm aware every of its fs function ends up calling crate::path::absolute(returns a Result<PathBuf>) or uefi_fs::File::from_path, which calls on crate::path::absolute anyways, so it made sense for me to make its fs functions take a PathBuf.

I did some other refactoring such as putting with_native_path functions within std/src/sys/path (except for unsupported.rs which has with_native_path implemented within its file). I thought it made sense to centralize the different with_native_path (aside from run_path_with_cstr can just use the alias with_native_path) functions there considering that's done for Windows. I also renamed the unsupported_backslash.rs file to solid.rs because it seems like solid is the only one that utilizes that file; the unix.rs file was renamed to common.rs because that's used by unix platforms, motor, and other miscellaneous platforms (motor platform has a different with_native_path implementation however).

If there's anything here that I should revert or change from platform to platform, just let me know.

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[asder8215]

To give a heads up, this PR might be better to wait on #158168 to merge in first, so that I can have with_native_path supported easily on set_perms_nofollow on all platforms.