Expose Program#isSourceFileFromExternalLibrary

[ajafff]

tslint --project . allows linting the whole project by creating a Program. But users don't want to lint anything imported from node_modules. TSLint tries to avoid linting dependencies by excluding *.d.ts files by default.
If allowJs is enabled in tsconfig.json, we need another way to exclude node_modules.

Program already has an internal method isSourceFileFromExternalLibrary, that does exactly what we need. Can you expose that API?