Preserve the SourceFile class attribute in dex2jar

[rushiiMachine]

Pretty simple, however useful for later decompilation with something like quiltflower

[Col-E]

That seems like it is already tracked:

• https://github.com/ThexXTURBOXx/dex2jar/blob/main/dex-reader-api/src/main/java/com/googlecode/d2j/node/DexClassNode.java#L31
• https://github.com/ThexXTURBOXx/dex2jar/blob/main/dex-reader/src/main/java/com/googlecode/d2j/reader/DexFileReader.java#L835

The test class AppWriterTest pulls from the test resource i_jetty.dex which contains source files. Looks like it works to me.