[EngSys] Use consistent file path handling in all JS code

Currently, we have a mix of file path handling in our JS code:

* APIs like "git diff" return posix paths even on Windows
* Many APIs accept posix paths on Windows
* Some APIs require OS-specific handling (e.g. path manipulation)

Ideally we would standardize on one of two approaches:

1. Nomalize all paths to posix for processing, only convert to os-specific when needed
2. Normalize all paths to os-specific as soon as possible (e.g. our "git diff" wrapper would return os-specific paths)

@timotheeguerin suggests option 1, since it's easier in practice and matches the TypeSpec repos.