v6.0.0-beta.6 release of PowerShellCore

Here are the major changes:

* Breaking change

- Make invalid argument error messages for `-File` and `-Command` consistent and make exit codes consistent with Unix standards (#4573)

* Engine updates and fixes

- Make resource loading to work with PowerShell SxS installation (#4139)
- Add missing assemblies to TPA list to make Pwrshplughin.dll work (#4502)
- Make sure running `powershell` starts instance of the current version of PowerShell. (#4481)
- Make sure we only use Unicode output by default on Nano and IoT systems (#4074)
- Enable `powershell -WindowStyle` to work on Windows. (#4573)
- Enable enumeration of COM collections. (#4553)

* General cmdlet updates and fixes

- Fix Web CmdLets `-SkipHeaderValidation` to work with non-standard User-Agent headers. (#4479 & #4512) (Thanks @markekraus)
- Add Certificate authentication support for Web CmdLets. (#4646) (Thanks @markekraus)
- Add support for content headers to Web CmdLets. (#4494 & #4640) (Thanks @markekraus)
- Add support for converting enums to string (#4318) (Thanks @KirkMunro)
- Ignore casing when binding PSReadline KeyHandler functions (#4300) (Thanks @oising)
- Fix `Unblock-File` for the case of a read-only file. (#4395) (Thanks @iSazonov)
- Use supported API to set Central Access Policy ID (CAPID) in SACL. (#4496)
- Make `Start-Trace` support paths that require escaping in the underlying APIs (#3863)
- Removing `#if CORECLR` enabled, `Enable-PSRemoting` and `Disable-PSRemoting` (#2671)
- Enable WSManCredSSP cmdlets and add tests. (#4336)
- Use .NET Core's implementation for ShellExecute. (#4523)
- Fix SSH Remoting handling of KeyFileParameter when the path must be quoted. (#4529)
- Make Web CmdLets use HTML meta charset attribute value, if present (#4338)
- Move to .NET Core 2.0 final (#4603)

* Build/test and code cleanup

- Add Amazon Linux Docker image and enable related tests. (#4393) (Thanks @DarwinJS)
- Make MSI verify pre-requisites are installed. (#4602) (Thank @bergmeister)
- Fixed formatting issues in build files. (#4630) (Thanks @iSazonov)
- Make sure `install-powershell.sh` installs latest powershell on macOS, even if an old version is cached in brew. (#4509) (Thanks @richardszalay for reporting.)
- Fixes install scripts issue for macOS. (#4631) (Thanks @DarwinJS)
- Many stability improvements to our nightly code coverage automation. (#4313 & #4550)
- Remove hash validation from nanoserver-insider Docker file, due to frequent changes. (#4498)
- Update to make Travis-CI daily build badge more reliable. (#4522)
- Remove unused build files, build code, and product code. (#4532, #4580, #4590, #4589, #4588, #4587, #4586, #4583, #4582, #4581)
- Add additional acceptance tests for PowerShellGet. (#4531)
- Only publish a NuGet of the full PowerShell core package on daily builds and not merge. (#4517)
- Update nanoserver-insider Docker file due to breaking changes in the base image. (#4555)
- Cleanup engine tests (#4551)
- Fix intermittent failures in filesystem tests (#4566)
- Add tests for
    - `New-WinEvent`. (#4384)
    - tab completion.  (#4560)
    - various types. (#4503)
    - CDXML CmdLets. (#4537)
- Only allow packaging of powershell, if it was built from a repo at the root of the file system named powershell. (#4569 & #4600)
- Update `Format-Hex` test cases to use -TestCase instead of foreach loops. (#3800)
- Added functionality to get code coverage for a single file locally. (#4556)

* Documentation

- Added Ilya (@iSazonov) as a Maintainer. (#4365)
- Grammar fix to the Pull Request Guide. (#4322)
- Add homebrew for macOS to install documentation. (#3838)
- Added a CodeOwner file. (#4565 & #4597)

* Cleanup `#if CORECLR` code

PowerShell 6.0 will be exclusively built on top of CoreCLR, so we are removing a large amount of code that's built only for FullCLR.
