v6.0.0-alpha.7 release of PowerShell

With 294 non-merge commits since v0.6.0, here are the major changes:

- Invoke-WebRequest and Invoke-RestMethod ported to PowerShell Core
- Set PSReadLine default edit mode to Emacs on Linux
- IsCore variable renamed to IsCoreCLR
- LocalAccounts and other Windows-only assemblies excluded on Linux
- PowerShellGet fully ported to Linux
- PackageManagement NuGet provider ported
- Write-Progress ported to Linux
- Get-Process -IncludeUserName ported
- Enumerating symlinks to folders fixed
- Bugs around administrator permissions fixed on Linux
- ConvertFrom-Json multi-line bug fixed
- Execution policies fixed on Windows
- TimeZone cmdlets added back; excluded from Linux
- FileCatalog cmdlets add back for Windows
- Get-ComputerInfo cmdlet added back for Windows
