make `checker.getAllPossiblePropertiesOfTypes` a public method

## Search Terms compiler, getAllPossiblePropertiesOfTypes, union, intersection, checker, Public API ## Suggestion Change the `checker.getAllPossiblePropertiesOfTypes` from private to public ## Use Cases This is the only easy way to calculate all the possible properties of an object and is used by the `completions` API to provide suggestion. I am currently using this API in https://github.com/styleguidist/react-docgen-typescript/pull/247 to document react props. ## Examples https://github.com/styleguidist/react-docgen-typescript/blob/master/src/parser.ts#L513 ## Checklist My suggestion meets these guidelines: * [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code * [x] This wouldn't change the runtime behavior of existing JavaScript code * [x] This could be implemented without emitting different JS based on the types of the expressions * [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.) * [x] This feature would agree with the rest of [TypeScript's Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals). BTW I would be happy to make these changes myself and submit a PR.