Make JsonEmitter more like HumanEmitter.

Use `derive(Setters)` to derive setters, and then change
`JsonEmitter::new` to only have the arguments that are always used.

compiler/rustc_errors/src/json/tests.rs

@@ -48,16 +48,10 @@ fn test_positions(code: &str, span: (u32, u32), expected_output: SpanTestData) {
         let output = Arc::new(Mutex::new(Vec::new()));
         let je = JsonEmitter::new(
             Box::new(Shared { data: output.clone() }),
-            None,
             sm,
-            None,
             fallback_bundle,
-            true,
+            true, // pretty
             HumanReadableErrorType::Short(ColorConfig::Never),
-            None,
-            false,
-            false,
-            TerminalUrl::No,
         );
 
         let span = Span::with_root_ctxt(BytePos(span.0), BytePos(span.1));