Fix net462/C# 7.3 build: avoid switch-expression recursive patterns

CS8370: recursive patterns require C# 8+; the net462 (PS 5.1) target
compiles with C# 7.3. Replaced the switch expression with an if/else
chain.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jessehouwing

Rules/UseCorrectCasingForDotSourcedFiles.cs

@@ -65,12 +65,19 @@ public override IEnumerable<DiagnosticRecord> AnalyzeScript(Ast ast, string file
                 }
 
                 Ast literalAst = commandAst.CommandElements[0];
-                string raw = literalAst switch
+                string raw;
+                if (literalAst is StringConstantExpressionAst constant)
                 {
-                    StringConstantExpressionAst constant => constant.Value,
-                    ExpandableStringExpressionAst expandable => expandable.Value,
-                    _ => null,
-                };
+                    raw = constant.Value;
+                }
+                else if (literalAst is ExpandableStringExpressionAst expandable)
+                {
+                    raw = expandable.Value;
+                }
+                else
+                {
+                    raw = null;
+                }
                 if (string.IsNullOrWhiteSpace(raw))
                 {
                     continue;