jextract: support extracting class members

[AbdAlRahmanGad]

closes: #835

[ktoso]

Thanks for adding snippets but we're not using them it seems -- can you add a small bit in FeaturesJextract.md around static methods somewhere:

+### Class methods
+
+Swift `class` methods and computed properties are imported as `static` methods on the generated Java wrapper type.
+
+@TabNavigator {
+   @Tab("Swift") {
+      @Snippet(path: "Snippets/ClassesSwift.swift", slice: "classMembers")
+   }
+   @Tab("Java (JNI)") {
+      @Snippet(path: "Snippets/MySwiftClassTest", slice: "classMembers")
+   }
+   @Tab("Java (FFM)") {
+      @Snippet(path: "Snippets/MySwiftClassTest", slice: "classMembers")
+   }
+}

And make sure we have the snippets for the usage as well

[ktoso]

I think we missed a condition in JNISwift2JavaGenerator.swift:175 in func supportedProtocolRequirements(of type: ExtractedNominalType) -> [ExtractedFunc] { maybe?

[ktoso]

Just a minor nit, I think these semantics will work out fine given how we generate code for the java side :)

[AbdAlRahmanGad]

I think we missed a condition in JNISwift2JavaGenerator.swift:175 in func supportedProtocolRequirements(of type: ExtractedNominalType) -> [ExtractedFunc] { maybe?

It's actually there, but maybe we should drop the condition since protocols doesn't support class members?

[ktoso]

Hm yeah, good question -- we could drop it there I think indeed; This PR is good though so let's land this and we can think about the latter :)