Fix handling of paths longer than 260 chars on Windows during export

[OmegaJak]

This addresses an issue brought to our attention by the modpack "Mimi Adventure", with the following repro steps, using version 0.6.0 of the app on Windows:

1. Create a profile using the modpack "Mimi Adventure" (https://modrinth.com/modpack/mimi-adventure)
2. Unpair the profile
3. Launch minecraft
4. Export the modpack with the "pfm" folder included as an override.
   This produces the following error:

Filesystem error: Path "\\\\?\\C:\\Users\\USER\\AppData\\Roaming\\com.modrinth.theseus\\profiles\\Mimi\\pfm\\cache\\resources\\assets\\pfm\\models\\block\\chair_classic\\stripped_luphie_flowering_pink_luphieclutteredmod_chair_classic\\stripped_luphie_flowering_pink_luphieclutteredmod_chair_classic_tucked.json" does not correspond to a profile

This error occurs because the path length is over 260 characters (Windows docs), which caused dunce::canonicalize (lib.rs line 187) to default to std::fs::canonicalize, which canonicalizes to a UNC path. This path begins with "\\?\", so when we try to strip the profile prefix (which isn't UNC) from the file's path, it fails.

My (somewhat clunky) solution is to use std::fs::canonicalize for both the profile path and the one we're canonicalizing, which should behave consistently regardless of length. The UNC path weirdness shouldn't matter to us here as we're only doing this if we can subtract the canonicalized profile path from the input path, canceling out the UNC prefix.

[OmegaJak]

We may want to consider using something like relative-path for these intermediate paths we use within Theseus, I'm guess it would take care of issues like this that are created by us doing our own relative path math.

[triphora]

I am incredibly impressed that you were able to find this.