v6.0.0-beta.2 release of PowerShellCore

Here are the major changes:

* Support backgrounding of pipelines with ampersand (`&`) (#3360)

- Putting `&` at the end of a pipeline will cause the pipeline to be run as a PowerShell job.
- When a pipeline is backgrounded, a job object is returned.
- Once the pipeline is running as a job, all of the standard `*-Job` cmdlets can be used to manage the job.
- Variables (ignoring process-specific variables) used in the pipeline are automatically copied to the job so `Copy-Item $foo $bar &` just works.
- The job is also run in the current directory instead of the user's home directory.
- For more information about PowerShell jobs, see [about_Jobs](https://msdn.microsoft.com/en-us/powershell/reference/6/about/about_jobs).

* Engine updates and fixes

- Crossgen more of the .NET Core assemblies to improve PowerShell Core startup time. (#3787)
- Enable comparison between a `SemanticVersion` instance and a `Version` instance that is constructed only with `Major` and `Minor` version values.
  This will fix some cases where PowerShell Core was failing to import older Windows PowerShell modules. (#3793) (Thanks to @mklement0!)

* General cmdlet updates and fixes

- Support Link header pagination in web cmdlets (#3828)
  - For `Invoke-WebRequest`, when the response includes a Link header we create a RelationLink property as a Dictionary representing the URLs and `rel` attributes and ensure the URLs are absolute to make it easier for the developer to use.
  - For `Invoke-RestMethod`, when the response includes a Link header we expose a `-FollowRelLink` switch to automatically follow `next` `rel` links until they no longer exist or once we hit the optional `-MaximumFollowRelLink` parameter value.
- Update `Get-ChildItem` to be more in line with the way that the *nix `ls -R` and the Windows `DIR /S` native commands handle symbolic links to directories during a recursive search.
  Now, `Get-ChildItem` returns the symbolic links it encountered during the search, but it won't search the directories those links target. (#3780)
- Fix `Get-ChildItem` to continue enumeration after throwing an error in the middle of a set of items.
  This fixes some issues where inaccessible directories or files would halt execution of `Get-ChildItem`. (#3806)
- Fix `ConvertFrom-Json` to deserialize an array of strings from the pipeline that together construct a complete JSON string.
  This fixes some cases where newlines would break JSON parsing. (#3823)
- Enable `Get-TimeZone` for macOS/Linux. (#3735)
- Change to not expose unsupported aliases and cmdlets on macOS/Linux. (#3595) (Thanks to @iSazonov!)
- Fix `Invoke-Item` to accept a file path that includes spaces on macOS/Linux. (#3850)
- Fix an issue where PSReadline was not rendering multi-line prompts correctly on macOS/Linux. (#3867)
- Fix an issue where PSReadline was not working on Nano Server. (#3815)
