Implement Null Coalescing and Null Coalescing assignment operators (PowerShell#10636)

Author: adityapatwardhan

src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

@@ -120,7 +120,10 @@ static ExperimentalFeature()
                     description: "New formatting for ErrorRecord"),
                 new ExperimentalFeature(
                     name: "PSUpdatesNotification",
-                    description: "Print notification message when new releases are available")
+                    description: "Print notification message when new releases are available"),
+                new ExperimentalFeature(
+                    name: "PSCoalescingOperators",
+                    description: "Support the null coalescing operator and null coalescing assignment operator in PowerShell language")
             };
             EngineExperimentalFeatures = new ReadOnlyCollection<ExperimentalFeature>(engineFeatures);
 

src/System.Management.Automation/engine/parser/Compiler.cs

@@ -279,9 +279,9 @@ internal static class CachedReflectionInfo
             typeof(InterpreterError).GetMethod(nameof(InterpreterError.NewInterpreterExceptionWithInnerException), StaticFlags);
 
         internal static readonly MethodInfo LanguagePrimitives_GetInvalidCastMessages =
-            typeof(LanguagePrimitives).GetMethod(nameof(LanguagePrimitives.GetInvalidCastMessages), StaticFlags);
+            typeof(LanguagePrimitives).GetMethod(nameof(LanguagePrimitives.GetInvalidCastMessages), staticFlags);
         internal static readonly MethodInfo LanguagePrimitives_IsNullLike =
-            typeof(LanguagePrimitives).GetMethod(nameof(LanguagePrimitives.IsNullLike), StaticPublicFlags);
+            typeof(LanguagePrimitives).GetMethod(nameof(LanguagePrimitives.IsNullLike), staticPublicFlags);
         internal static readonly MethodInfo LanguagePrimitives_ThrowInvalidCastException =
             typeof(LanguagePrimitives).GetMethod(nameof(LanguagePrimitives.ThrowInvalidCastException), StaticFlags);
 

src/System.Management.Automation/engine/parser/token.cs

@@ -1063,8 +1063,8 @@ public static class TokenTraits
             /*                  Shr */ "-shr",
             /*                Colon */ ":",
             /*         QuestionMark */ "?",
-            /*    Reserved slot 3   */ string.Empty,
-            /*    Reserved slot 4   */ string.Empty,
+          /* QuestionQuestionEquals */ "??=",
+            /*     QuestionQuestion */ "??",
             /*    Reserved slot 5   */ string.Empty,
             /*    Reserved slot 6   */ string.Empty,
             /*    Reserved slot 7   */ string.Empty,