Add support for summary output

[ncalteen]

This PR adds support for job summary output. If enabled via the GITHUB_STEP_SUMMARY env var, it will write any summary output to the designated file path.

A few other housecleaning items were included:

• Add the expanded list of available GitHub environment variables to reference in .env.example
• Move feature information to separate files in docs/
• Add more tests :)
• Add support for toWin32Path, toPlatformPath, and toPosixPath
• Add docs on supported actions/toolkit packages/functions