Properly handle Function and TSMethodSignature TS interfaces and type literals

[chandlerprall]

Summary

Noticed that the docs showed proptype warnings for EuiCopy's children, even though everything was typed & passed correctly. Discovered that the Function identifier and method signatures were not being detected by the TS->proptype script. Added a test case to cover

{
  foo(): ReactElement;
  bar?(arg: number): string;
  fizz: Function;
  buzz?: (arg: boolean) => string;
}

(which should all result in either PropTypes.func or PropTypes.func.isRequired)

in both interface and type definitions, and confirmed EuiCopy's proptype is now correct.

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode

• Any props added have proper autodocs
  - [ ] Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
  - [ ] This was checked against keyboard-only and screenreader scenarios
  - [ ] This required updates to Framer X components

[pugnascotia]

I build the branch and the generated prop type for children looks correct.