Add missing `export type` notation for unsupported types

[cmaster11]

Previously, a definition like

export const TestSchema = Joi.function().meta({ className: 'Test' });

was exporting broken TS code:

(...args: any[]) => any;

It now exports:

export type Test = (...args: any[]) => any;

Note: this will conflict on merge with https://github.com/mrjono1/joi-to-typescript/pull/400. I'll take care of it if merged. Probably one test will fail.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4366519) 98.69% compared to head (5bedda5) 98.69%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #401 +/- ## ======================================= Coverage 98.69% 98.69% ======================================= Files 9 9 Lines 535 537 +2 Branches 206 207 +1 ======================================= + Hits 528 530 +2 Misses 7 7 ```

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[cmaster11]

@mrjono1 test fixed now :)