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
> what happens if you do pathComponents on a string that starts with /?

```
[@"this/that.js" pathComponents]

(
    this,
    "that.js"
)

[@"/this/that.js" pathComponents]

(
    "/",
    this,
    "that.js"
)
```

You could test the first item in pathComponents, but you'd still be testing if it's equal to @"/".

> can't you just do charAtIndex here?

You can, but comparing unichars is ugly, and I bet it uses `substringWithRange:` internally anyway.

```
const unichar pathSeparator = *(const unichar *)[@"/" cStringUsingEncoding:NSUnicodeStringEncoding];

NSLog(@"%i", [@"this/that.js" characterAtIndex:0] == pathSeparator);  // 0			
NSLog(@"%i", [@"/this/that.js" characterAtIndex:0] == pathSeparator); // 1	
```