fixes #396

Currently, the @luaIterator directive is applied to a function that generates an iterator. This causes issues with tracking that a type is a lua iterator for use in for...of loops.

declare namespace string {
    /** @luaIterator @tupleReturn */
    export function gmatch(s: string, pattern: string): Iterable<string[]>;
}

const iter = string.gmatch("foobar", ".");
for (const [c] of iter) {} // 'iter' is seen as a regular Iterable and won't transpile correctly here

This PR changes the @luaIterator so that it is applied to the type itself:

declare namespace string {
    /** @luaIterator @tupleReturn */
    export interface GmatchResult extends Iterable<string[]> {}
    export function gmatch(s: string, pattern: string): GmatchResult;
}

This allows that iterator to be passed around freely without losing the directive. It will correctly transpile to a native for...in loop regardless of its source.

Note this is a breaking change! But the old way was error-prone and inconsistent with how other directives work, so I believe it's worth requiring declarations to be updated.