v6.0.0-alpha.18 release of PowerShellCore

Here are the major changes:

* Progress Bar

We made a number of fixes to the progress bar rendering and the `ProgressRecord` object that improved cmdlet performance and fixed some rendering bugs on non-Windows platforms.

- Fix a bug that caused the progress bar to drift on Unix platforms. (#3289)
- Improve the performance of writing progress records. (#2822) (Thanks to @iSazonov!)
- Fix the progress bar rendering on Unix platforms. (#3362) (#3453)
- Reuse `ProgressRecord` in Web Cmdlets to reduce the GC overhead. (#3411) (Thanks to @iSazonov!)

* Cmdlet updates

- Use `ShellExecute` with `Start-Process`, `Invoke-Item`, and `Get-Help -Online` so that those cmdlets use standard shell associations to open a file/URI.
  This means you `Get-Help -Online` will always use your default browser, and `Start-Process`/`Invoke-Item` can open any file or path with a handler.
  (Note: there are still some problems with STA threads.) (#3281, partially fixes #2969)
- Add `-Extension` and `-LeafBase` switches to `Split-Path` so that you can split paths between the filename extension and the rest of the filename. (#2721) (Thanks to @powercode!)
- Implement `Format-Hex` in C# along with some behavioral changes to multiple parameters and the pipeline. (#3320) (Thanks to @MiaRomero!)
- Add `-NoProxy` to web cmdlets so that they ignore the system-wide proxy setting. (#3447) (Thanks to @TheFlyingCorpse!)
- Fix `Out-Default -Transcript` to properly revert out of the `TranscribeOnly` state, so that further output can be displayed on Console. (#3436) (Thanks to @PetSerAl!)
- Fix `Get-Help` to not return multiple instances of the same help file. (#3410)

* Interactive fixes

- Enable argument auto-completion for `-ExcludeProperty` and `-ExpandProperty` of `Select-Object`. (#3443) (Thanks to @iSazonov!)
- Fix a tab completion bug that prevented `Import-Module -n<tab>` from working. (#1345)

* Cross-platform fixes

- Ignore the `-ExecutionPolicy` switch when running PowerShell on non-Windows platforms because script signing is not currently supported. (#3481)
- Standardize the casing of the `PSModulePath` environment variable. (#3255)

* JEA fixes

- Fix the JEA transcription to include the endpoint configuration name in the transcript header. (#2890)
- Fix `Get-Help` in a JEA session. (#2988)
