File support

[nsalerni]

Refs: #250

Issue #250

https://github.com/awslabs/llrt/issues/250

Description of changes

Implemented the File interface: https://developer.mozilla.org/en-US/docs/Web/API/File. This is a pre-requisite for implementation of FormData.

Checklist

• [x] Created unit tests in tests/unit and/or in Rust for my feature if needed
• [x] Ran make fix to format JS and apply Clippy auto fixes
• [x] Made sure my code didn't add any additional warnings: make check
• [x] Updated documentation if needed (API.md/README.md/Other)

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