refactor: lifts toPosix to its own file, removes circular dependency between stack_utils and source_map_utils

[cacieprins]

• Closes

Additional details

Lifts the function from stack_utils that source_map_utils was using to its own file. This enables the removal of the stack_utils.ts <-> source_map_utils cycle. Vitest unit tests are added as well.

This utility is recreated several times across the repo. Perhaps a single third party npm package, like unixify or normalize-path would be helpful.

Steps to test

How has the user experience changed?

PR Tasks

• [X] Have tests been added/updated?
• [N/A] Has a PR for user-facing changes been opened in cypress-documentation?
• [N/A] Have API changes been updated in the type definitions?