Update type definitions

[shawnmcknight]

This PR makes a few changes to the TS type definitions:

1. I found that a few type definitions were not being exported and thus were inaccessible. I believe the existing format of how things were being exported was a contributing factor, so I adjusted every type and interface to export at the location of the definition and eliminated the exports at the end of the declaration file. Now everything should be accessible and any new definitions added, provided they follow this pattern, will end up exported without having to add them to the follow-up exported object.
2. ValidatorConstructorOptions was declared as a type, but it would be better declared as an interface. I have adjusted this type.
3. The third change I am not 100% sure on, so would love some feedback. In the CheckerFunctionV1 and CheckerFunctionV2 function types, there was a schema parameter which was set as ValidationSchema. When inspecting the parameters passed to the custom function, it did not appear to line up with ValidationSchema and instead was the rule's schema. This seemed to represent what the code was actually doing as it passes rule.schema to these functions. While I definitely think ValidationSchema is incorrect, I'm unsure if what should be passed in this parameter is ValidationRule (which is union ValidationRuleObject | ValidationRuleObject[] | ValidationRuleName) or a subset of that union. In my testing, I only ever saw ValidationRuleObject (single, not array) being passed in, but I'll admit I'm quite unsure of what can actually get passed into the custom function.

[coveralls]

Coverage remained the same at 100.0% when pulling 58b37fc on shawnmcknight:improve-types into c6b040a on icebob:master.

[icebob]

@erfanium Could you help with it? I'm not a TS guy.

[erfanium]

@shawnmcknight Thanks! Change are OK to me!