[WIP] Refactor FileSystem for better tree-shaking

[dmichon-msft]

Summary

Structures the FileSystem collection of functions as a namespace internally and moves each implementation to its own file so that ESM-aware bundlers (like webpack or esbuild) can tree-shake them and split them into separate async chunks if needed, while preserving the API contract.

Details

Each implementation receives its own file, which both reduces the amount of code that needs to be loaded for unit tests and allows ESM bundlers like webpack or esbuild to separate implementations into different chunks if they are used at different times.

How it was tested

api-extractor is currently not happy. Rest of the package (which includes use of FileSystem) builds fine without changes.

[iclanton]

Note: PR #3337 has renamed our GitHub master branch to main. This should not affect your pull request, but we recommend to redo your git clone to avoid confusion with the old branch name.