Library setup (#1)

This PR contains the work done to setup the library:
* Defined products, dependencies, and targets for the `Package` file;
* Restructured the folder structure of the library and test targets to start development work;
* Created the `DocC` documentation catalog in the library target;
* Added the `CONTRIBUTORS` and `LICENSE` files;
* Added `README` file.

Reviewed-on: #1
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Tests/MarvelService/Cases/marvel_serviceTests.swift

@@ -0,0 +1,6 @@
+import Testing
+@testable import marvel_service
+
+@Test func example() async throws {
+    // Write your test here and use APIs like `#expect(...)` to check expected conditions.
+}