v6.0.0-alpha.15 release of PowerShellCore

Here are the major changes:

- Use parens around file length for offline files
- Fix issues with the Windows console mode (terminal emulation) and native executables
- Fix error recovery with `using module`
- Report `PlatformNotSupported` on IoT for Get/Import/Export-Counter
- Add `-Group` parameter to `Get-Verb`
- Use MB instead of KB for memory columns of `Get-Process`
- Add new escape character for ESC: `` `e``
- Fix a small parsing issue with a here string
- Improve tab completion of types that use type accelerators
- `Invoke-RestMethod` improvements for non-XML non-JSON input
- PSRP remoting now works on CentOS without addition setup
