Adds plugin hooks for file validation

[TwitchBronBron]

• Adds beforeFileValidate plugin hook. While currently the afterFileParse event happens at roughly the same time, adding this new hook makes it more approachable to extension authors (i.e. they don't have to read the docs), and also guards against issues that may occur if we add events between afterFileParse and beforeFileValidate in the future. It also makes things more clear when doing pre-validate work, then doing post-validate cleanup.
• Adds onFileValidate plugin hook which is how where plugins should hook in when contributing their own file validations.

Since the v1 plugin system event parameters use a single event object (instead of 1...n parameters), all newly added plugin events in master will also follow that style to minimize impacts of plugins migrating to v1.