Lowercase drive letter on Windows

[phated]

VSCode seems to provide file paths with a lowercase drive letter (actually, it's even more weird, file:///c:/Users/User/...) and we do a bunch of normalization in order for the Grain compiler to work. For Packaged Grain, it fails if the drive letter is lowercase, so in #45 I needed to add a normalization that capitalized it. We should figure out the more proper fix.