Expose ts.matchFiles as public API to make implementing readDirectory easier.

Problem: When implementing a custom host that virtualizes the file system, the `readDirectory` method is cumbersome to implement in a way that matches tsc behavior. If you look at the hosts within typescript, the `matchFiles` method was added to unify behavior with `readDirectory` between hosts. While `matchFiles` is exported, it isn't in the type definition files so I'm assuming it is "private" API. Either the host should be able to provide a `getFileSystemEntries(path: string) => { files: string[], directories: string[] }` to get a `readDirectory` implementation or expose `matchFiles`. I'm working on improving the `broccoli-typescript-compiler` broccoli plugin to have more parity with tsc and VS Code, but I need to virtualize the FS, since broccoli builds from tmp directory and I want the node_modules/@types discovery, tsconfig extends, etc all to work, I need to pretend the tmp dir is inside the project. Thanks