BohemianCoding/Sketch#3319 Script import: Add support for absolute and ~ paths

Import paths can be in the format:

    relative/to/script.js
    /absolute/path/to/script.js
    ~/relative/to/home/script.js

Ref BohemianCoding/Sketch#3319
Or better - what happens if you do pathComponents on a string that starts with /?
If you get back an array where the first item is an empty string, that would be ideal...