Added cmdlet Get-ComputerName

[johlju]

Pull Request (PR) description

• Added cmdlet Get-ComputerName which can be used to returns the computer
  name cross-plattform. The variable $env:COMPUTERNAME does not exist
  cross-platform which hinders development and testing on macOS and Linux.
  Instead this cmdlet can be used to get the computer name cross-plattform.

This Pull Request (PR) fixes the following issues

None.

Task list

• Added an entry to the change log under the Unreleased section of the
  file CHANGELOG.md. Entry should say what was changed and how that
  affects users (if applicable), and reference the issue being resolved
  (if applicable).
• Documentation added/updated in README.md.
• Comment-based help added/updated for all new/changed functions.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Community Testing Guidelines.
• Integration tests added/updated (where possible). See
  DSC Community Testing Guidelines.
• New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[gaelcolas]

Looks good