[metal-soy-critic] Allow for use as a Library

[jbalsas]

From Allow for use as a Library by @mthadley:

We should export some functions that allows using metal-soy-critic as a library. We will probably need to update some of the return types so that they make sense for more generic use cases.

We can also consider adding a JSON output option. This would be useful for integration with tools like linters, continuous integration, etc.

[jbalsas]

This is the pre-requisite for integration into liferay/liferay-frontend-source-formatter#21