v2.0 Fixes, Security, Lamdera compatibility

The main changes of this version are the following:

• Breaking: I fixed an error in the json report output, where testsCompleted should have been testCompleted instead. It may not have been impacting many people, but it's still a breaking change, so here we go 2.0.0.
• Other bug fixes: Big thanks to @jjant for fixing the package.json file specifying that the code generated by elm-test-rs is commonjs.
• Security: Some indirect dependency had a security issue in it's usage of time (RUSTSEC-2020-0071). This never occurred with how the crate is used in elm-test-rs, but just to be sure, I removed the dependency.
• Quality of life: elm-test-rs now has better compatibility with Lamdera. This is convenient as recent versions of elm-test have an incompatibility with it due to an incompatibility between elm-json and Lamdera.
• Future-proofness: Some options have been added to smooth a potential future integration in the language server.