Support target and src being on different drives (for example, E:, C:)

[cspotcode]

Users of npm-pwsh reported cmd-shim can't link across drives.
cspotcode/npm-pwsh#18
npm-pwsh is currently using cmd-shim, but I'd like to switch to @zkochan/cmd-shim. Unfortunately both libraries exhibit this limitation.

The problem can be generalized to handling when the target path must be absolute instead of relative.

This can be accomplished by modifying these 2 lines:
https://github.com/pnpm/cmd-shim/blob/master/index.js#L252
https://github.com/pnpm/cmd-shim/blob/master/index.js#L257

... to check if target is an absolute path. If it is, it means we're linking to a different drive letter, so use target verbatim. If not, prepend with %~dp0 just like the current behavior.

Related: npm#21