Test-SqlDscRSInstalled: Add command

[johlju]

Pull Request (PR) description

• Test-SqlDscRSInstalled to test whether an instance is installed or not
  (issue #2078).

This Pull Request (PR) fixes the following issues

• Fixes Test-SqlDscRSIsInstalled: New command proposal #2078

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).
• Resource documentation updated in the resource's README.md.
• Resource parameter descriptions updated in schema.mof.
• Comment-based help updated, including parameter descriptions.
• Localization strings updated.
• Examples updated.
• Unit tests updated. See DSC Community Testing Guidelines.
• Integration tests updated (where possible). See DSC Community Testing Guidelines.
• Code changes adheres to DSC Community Style Guidelines.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94%. Comparing base (a8befef) to head (9f2617b).
Report is 16 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff          @@
##           main   #2079   +/-   ##
====================================
  Coverage    94%     94%           
====================================
  Files       103     104    +1     
  Lines      8083    8090    +7     
====================================
+ Hits       7656    7663    +7     
  Misses      427     427           

Flag	Coverage Δ
unit	∅ <100%> (∅)

Files with missing lines	Coverage Δ
source/Public/Test-SqlDscRSInstalled.ps1	100% <100%> (ø)

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[johlju]

Reviewed 6 of 7 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johlju)