ffi 2.4 xfail: #extends Alias; body item in #jni_class

`Window :: #jni_class("...") { #extends View; ... }` should parse,
today's 2.3 parser doesn't recognise `#extends` as a token and the
body loop reports "expected method name". The make-green follow-up
adds `hash_extends`/`hash_implements` lexer tokens, refactors
`JniClassDecl.methods` into a `members` tagged union, and dispatches
in the body loop on the leading token.

tests/expected/ffi-jni-class-04-extends.exit

@@ -0,0 +1 @@
+1

tests/expected/ffi-jni-class-04-extends.txt

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+/Users/agra/projects/sx/examples/ffi-jni-class-04-extends.sx:18:5: error: expected method name in '#jni_class' body