Fix jstype = JS_STRING field option

There was a bug in  message-interface-generator.ts:

When the field option jstype = JS_STRING was set, the interface would still have a bigint property.

This problem only surfaces if the plugin parameter long_type_string is *not* set.

packages/plugin/src/code-gen/message-interface-generator.ts

@@ -287,7 +287,7 @@ export class MessageInterfaceGenerator {
             case rt.ScalarType.UINT64:
             case rt.ScalarType.FIXED64:
             case rt.ScalarType.SINT64:
-                switch (longType || this.options.normalLongType) {
+                switch (longType ?? rt.LongType.STRING) {
                     case rt.LongType.STRING:
                         return ts.createKeywordTypeNode(ts.SyntaxKind.StringKeyword);
                     case rt.LongType.NUMBER: