Implement Null Coalescing and Null Coalescing assignment operators

[adityapatwardhan]

PR Summary

Implement the Null Coalescing ?? and Null Coalescing Assignment ??= operators.

PR Context

The operators are discussed in the issue #3240
This PR addresses part of RFC PowerShell/PowerShell-RFC#223

The PR is marked as a Breaking Change due to changes in the TokenFlags enum. The changes were made to include the new TokenFlag - BinaryPrecedenceCoalesce. While making this change, it was also decided to create more space in the BinaryPrecedence section of the enum for future binary operators. The BinaryPrecedenceMask was also changed from 0x07 to 0x0f. The order of precedence is not changed. We expected it can cause a breaking change for binary modules as C# treats enums as constants. Though, it is a breaking change we expect the impact to be pretty low as the usage of precedence token flags should be fairly low.

PR Checklist

• PR has a meaningful title
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• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
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• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed: Add documentation for ?? ??= ?. and ?[] operators MicrosoftDocs/PowerShell-Docs#4925
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
  • OR
  • I have considered the user experience from a tooling perspective and enumerated concerns in the summary. This may include:
    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).
      • Issue opened Possible update to syntax highlighting for ?? ??= ?. ?[] operators EditorSyntax#185