add missing import file extensions, add @TODO comments for refactoring

[Connum]

Description

adds missing import file extensions

Motivation and Context

While reviewing #535, I noticed that some imports were still missing the file extension that we need now after changing the build system and allowing the library to work in module form in browser context. While I was at it, I also added TODO comments for something we need to address after #564 is merged.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I did npm run test and all tests passed green (including code styling checks).
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the README accordingly.
• I have read the CONTRIBUTING document.