Add JSON5/SJSON Comments When Marshalling

Allows user-facing JSON5/SJSON to have comments explaining field usage.
`json.Marshal_Options.pretty` must be enabled since we only use single-line comments (not to mention it wouldn't be terribly useful without `pretty` set anyways).
We don't escape anything, so `\n` will display as "\n", but you're still able to enter in a proper newline character and it'll be displayed on multiple lines.

core/encoding/json/marshal.odin

@@ -413,6 +413,12 @@ marshal_to_writer :: proc(w: io.Writer, v: any, opt: ^Marshal_Options) -> (err:
 				}
 
 				opt_write_iteration(w, opt, first_iteration) or_return
+
+				if opt.pretty {
+					comment := reflect.struct_tag_get(reflect.Struct_Tag(info.tags[i]), "jsoncomment")
+					opt_write_comment(w, opt, &comment) or_return
+				}
+
 				first_iteration = false
 				if json_name != "" {
 					opt_write_key(w, opt, json_name) or_return
@@ -533,6 +539,26 @@ marshal_to_writer :: proc(w: io.Writer, v: any, opt: ^Marshal_Options) -> (err:
 	return
 }
 
+// Newlines are split into multiple comment lines
+opt_write_comment :: proc(w: io.Writer, opt: ^Marshal_Options, comment: ^string) -> (err: io.Error) {
+	if comment^ == "" {
+		return nil
+	}
+
+	switch opt.spec {
+	case .JSON5, .MJSON:
+		for line in strings.split_iterator(comment, "\n") {
+			io.write_string(w, "// ") or_return
+			io.write_string(w, line) or_return
+			io.write_rune(w, '\n') or_return
+			opt_write_indentation(w, opt) or_return
+		}
+	case .JSON: return nil
+	}
+
+	return nil
+}
+
 // write key as quoted string or with optional quotes in mjson
 opt_write_key :: proc(w: io.Writer, opt: ^Marshal_Options, name: string) -> (err: io.Error)  {
 	switch opt.spec {