PR Summary

Add support to ternary operator <condition> ? <if-true> : <if-false>. It results in a TernaryExpressionAst.
Address #3239
RFC: PowerShell/PowerShell-RFC#218
The old draft PR where a lot of discussions happened: #10161

The ternary operator behaves like the simplified if-else statement. The condition-expression will always be evaluated, and its result will be converted to boolean to determine which branch will be evaluated next:

• if-true-expression will execute if the condition's result is evaluated as true
• if-false-expression will execute if the condition's result is evaluated as false

PR Context

Ternary operator has lower precedence than binary operator, so you can write $a -eq $b ? "Hello World" : [int]::MaxValue.
In order to make the number constant expression work naturally with ternary operators, such as in return ${succeed}?0:1, characters ? and : are made start-new-token chars conditionally when scanning for numbers.
When it's known for sure that we are expecting an expression, allowing a generic token like 123? is not useful and bound to result in parsing errors.
In those cases, we will force to start a new token upon seeing characters ? and : when scanning for a number, so that that number constant expressions can work with ternary operators more intuitively.

Note that, the implicit line continuance for ? support was reverted in the latest code change based on the feedback I received within the team.

PR Checklist

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• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s): PSTernaryOperator
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed: Add documentation for the ternary operator MicrosoftDocs/PowerShell-Docs#4648
• Testing - New and feature
  • N/A or can only be tested interactively
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  • 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.
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  • 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).