Set up pre-commit for running hooks and tasks such as linting.

[jacksonj04]

pre-commit is a cross-language framework for managing pre-commit hooks. This PR adds the necessary configuration and documentation changes, and then whips existing files into shape.

Specifically, this is configured to:

• Make sure lines don't have trailing whitespace
• Make sure the last line in a file is empty
• Check JSON and YAML files are syntactically valid
• Prevent files over 500kb (configurable) from being committed
• Run the prettier code formatter over scss, yaml, markdown and js files
• Run dotnet format across the codebase

Developers can install pre-commit in several ways, including through pip, homebrew and conda. Once that's done, running pre-commit install in the directory will set up the necessary git hooks.

[jacksonj04]

@dynamictulip @lookupdaily @jkempster34 we should investigate doing this with something like https://alirezanet.github.io/Husky.Net/