Fix: Label Functions and Methods declartions as Ambient (fixes #162)

[soda0289]

Ambient functions do not have a body and will cuase rules to throw
an exception. We use the types TSAmbientFunctionExpression and
TSAmbientMethodDefinition.

[eslintbot]

LGTM

[soda0289]

I noticed in FunctionDeclaration we use the type DeclareFunction when defining ambient function. Should we use TSAmbientFunctionDeclaration as well?

[eslintbot]

LGTM

[soda0289]

I made the change for the node DeclareFunction. It is now TSAmbientFunctionDeclaration to make it align with this pull request. At the very least we should prefix it with TS unless there is another rule that uses DeclareFunction.

[eslintbot]

LGTM

[Pajn]

When I looked at the test output of this declare class test for #207 I noticed that there is nothing on the class node that make it possible to see that it is declared. That info would be great for prettier so that it can now if it should print declare or not :)
It can of course be added in a different PR, just saw it here.

[soda0289]

@Pajn Yes we do need to label ambient classes. I was thinking of changing the type from Class to TSAmbientClass this way some eslint rules won't crash as they are not aware of empty body methods, which declared classes all have. This would break current linting rules that are meant for classes but it is better then having the crash.

[JamesHenry]

I'm so sorry it's taken so long to address these PRs @soda0289, so happy to have you on the team now! Let's take stock as soon as we merge in all the breaking changes and release v3!