Replace "/" with path.sep and use other Node path utilities to improve Windows support

[Mrcavas]

On Windows, the path separator is \ instead of / on UNIX systems. This means that the CLI is currently broken on Windows entirely.
To improve cross-platform support, please use the appropriate Node.js path utilities: path.sep instead of "/", check if the path is absolute with path.isAbsolute instead of .startsWith("/"), etc.

I haven’t checked the entire codebase, but some files that likely need changes include

• packages/build/src/utils/FileHelper.ts
• packages/sourcing/src/files/FileManager.ts
• packages/sourcing/src/modules/LocalModuleLocator.ts
• and others.