use cross-platform paths for migration

[galister]

Issue:

On platforms other than Windows, the migration process creates LTCGI.cginc and gizmo files outside of the Assets folder.

Dependent shaders will error out due to the include path not pointing to a valid file.

drwxr-xr-x 10 op op  4096 Dec  9 10:40  ./
drwxr-xr-x 13 op op  4096 Dec  8 19:57  ../
drwxr-xr-x 19 op op  4096 Dec  8 23:11  Assets/
-rw-r--r--  1 op op 11809 Dec  8 20:30 'Assets\Gizmos\LTCGI_Screen_Gizmo.png'
-rw-r--r--  1 op op    56 Dec  8 20:30 'Assets\_pi_\_LTCGI\Shaders\LTCGI.cginc'
-rw-r--r--  1 op op   403 Dec  8 20:03  Avatars22.sln
drwxr-xr-x 15 op op  4096 Dec  9 10:41  Library/
drwxr-xr-x  2 op op  4096 Dec  9 10:41  Logs/
drwxr-xr-x  7 op op  4096 Dec  8 22:53  Packages/
drwxr-xr-x  3 op op  4096 Dec  9 10:41  ProjectSettings/
drwxr-xr-x  4 op op  4096 Dec  9 10:45  Temp/
drwxr-xr-x  3 op op  4096 Dec  9 10:38  UserSettings/

Proposed solution

1. Use Path.Combine instead of hardcoded path separators
2. Use Directory.CreateDirectory to create the folder structure recursively. (Using AssetDatabase is not necessary, as we're not injecting any metadata.)

Tested

On Linux, but the behavior is expected to be the same on all platforms.

[PiMaker]

Had to double-check that Directory.CreateDirectory creates the full hierarchy if multiple folders are missing, but this looks to be correct.

Thank you for the contribution! I'll make sure to give it a quick test on Windows too and ship it with the next release (though I don't know when I'll have time for that yet).