Gradually deprecate running swift-format without input paths; require '-' for stdin in the future

Author: TTOzzi

Sources/swift-format/Frontend/Frontend.swift

@@ -97,7 +97,22 @@ class Frontend {
 
   /// Runs the linter or formatter over the inputs.
   final func run() {
-    if lintFormatOptions.paths.isEmpty {
+    if lintFormatOptions.paths == ["-"] {
+      processStandardInput()
+    } else if lintFormatOptions.paths.isEmpty {
+      diagnosticsEngine.emitWarning(
+        """
+        Running swift-format without input paths is deprecated and will be removed in the future.  
+
+        Please specify one of the following:
+
+        - Use - to read from stdin (e.g., cat MyFile.swift | swift-format -)
+        - Provide a directory path (e.g., swift format --recursive MyDirectory/)
+        - Provide a specific Swift source file (e.g., swift format MyFile.swift)
+
+        For more information, use the --help option.
+        """
+      )
       processStandardInput()
     } else {
       processURLs(