Support system-dependent path separators

Update `StandardConfigDataLocationResolver` so that it recognizes
both `/` and `File.separator` suffixes as directories.

Prior to this commit, working with directories on Windows was awkward
since the path separator is `\`. If a directory were specified in the
form `c:\config\`, an exception was raised complaining it did not end
with '/'.

See gh-24490

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/config/StandardConfigDataLocationResolver.java

@@ -26,6 +26,7 @@ import java.util.List;
 import java.util.Set;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import java.io.File;
 
 import org.apache.commons.logging.Log;
 
@@ -212,7 +213,7 @@ public class StandardConfigDataLocationResolver
 			}
 		}
 		throw new IllegalStateException("File extension is not known to any PropertySourceLoader. "
-				+ "If the location is meant to reference a directory, it must end in '/'");
+				+ "If the location is meant to reference a directory, it must end in '/' or File.separator");
 	}
 
 	private String getLoadableFileExtension(PropertySourceLoader loader, String file) {
@@ -225,7 +226,7 @@ public class StandardConfigDataLocationResolver
 	}
 
 	private boolean isDirectory(String resourceLocation) {
-		return resourceLocation.endsWith("/");
+		return resourceLocation.endsWith("/") || resourceLocation.endsWith(File.separator);
 	}
 
 	private List<StandardConfigDataResource> resolve(Set<StandardConfigDataReference> references) {