feature: add support for Windows

[byCedric]

This change includes a couple of changes, to make Atlas run smoothly on Windows machines.

1. The module/folder paths in the URLs are now all relative to the bundle's sharedRoot. This helps us not having to care about potential C:\ prefixes.
2. All AtlasModule and AtlasModuleRef objects now have absolutePath and relativePath (instead of path). The absolutePath is formatted in the system's dialect, so it can be looked up in Metro if necessary. The relativePath is relative to the bundle's sharedRoot and notated in POSIX.
3. The base64 ID when running Atlas during development is now shorter, and using the relative path of the entry file (using sharedRoot).