Implemented the FileService service in the library target.

Test/Sources/Cases/Internal/Services/FileServiceTests.swift

@@ -0,0 +1,7 @@
+import Testing
+
+@testable import DoxyLibrary
+
+@Suite("FileService", .tags(.service))
+struct FileServiceTests {
+}

Test/Sources/Helpers/Extensions/Tag+Definitions.swift

@@ -8,5 +8,6 @@ extension Tag {
     @Tag static var enumeration: Tag
     @Tag static var middleware: Tag
     @Tag static var provider: Tag
+    @Tag static var service: Tag
     
 }