Add profiles for PS 7.0.0-preview.3

[rjmholt]

PR Summary

Adds profiles for PowerShell 7.0.0-preview.3 for Ubuntu 18.04, Windows Server 2019 and Win10 1809, in the optional_profiles folder.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[bergmeister]

Please update those docs, otherwise looks ok:

• https://github.com/PowerShell/PSScriptAnalyzer/blob/master/RuleDocumentation/UseCompatibleTypes.md
• https://github.com/PowerShell/PSScriptAnalyzer/blob/master/RuleDocumentation/UseCompatibleCommands.md

Unrelated to this PR we should exchange the 6.1 profiles with 6.2 profiles because 6.1 goes out of support this month. Do we want this as part of 1.18.3 (which raised the minimum PSCore version to to be used to 6.2)

[rjmholt]

Please update those docs, otherwise looks ok

We currently don't mention the optional profiles in those docs, but yes I think we should document those.

Do we want this as part of 1.18.3

If we're willing to do more things in 1.18.3, then I'll try to get 6.2 profiles in in the next couple of days

[rjmholt]

Going to close this since I'm updating other profiles