JS build: fix TypeError when null occurs in YAML input

Fixes KSC "crashes" like this (this stack trace is from
https://ide.kaitai.io/devel/):

```
TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at new $c_sjs_js_WrappedDictionary$DictionaryIterator (https://ide.kaitai.io/devel/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:56693:69)
    at $c_sjs_js_WrappedDictionary.iterator__sc_Iterator (https://ide.kaitai.io/devel/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:108532:10)
    at $f_scm_Growable__addAll__sc_IterableOnce__scm_Growable (https://ide.kaitai.io/devel/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:43131:21)
    at $c_sci_MapBuilderImpl.addAll__sc_IterableOnce__sci_MapBuilderImpl (https://ide.kaitai.io/devel/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:70719:192)
    at $c_sci_Map$.from__sc_IterableOnce__sci_Map (https://ide.kaitai.io/devel/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:49082:26)
    at $c_Lio_kaitai_struct_format_JavaScriptKSYParser$.yamlJavascriptToScala__O__O (https://ide.kaitai.io/devel/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:17228:35)
    ...
```

I think that this is an error that Web IDE users see relatively often
when they're in the middle of writing a .ksy spec directly in the Web
IDE's built-in editor. Web IDE recompiles the .ksy spec automatically
while the user types, so it's likely that there will be an attempt to
compile a non-well-formed .ksy spec like this:

```ksy
meta:
  id: js_cannot_convert_null
instances:
  foo:
    # NOTE: the following line "value:" is equivalent to "value: null" in YAML
    value:
```

Notice that the `value:` line is unfinished, but the Web IDE doesn't
know that, so it compiles this spec anyway. As a result, the KSC crashes
with the stack trace posted above, because the `yamlJavascriptToScala`
wasn't prepared for `null` or `undefined` and tried to treat these
values as if they were JS objects (i.e. "dictionaries"), which doesn't
work. This is fixed in this commit, so the new error message in the Web
IDE for the above .ksy snippet looks like this:

```
(main): /instances/foo/value:
	error: expected string, got null
```

This is consistent with the error message printed by the JVM build:

```console
$ kaitai-struct-compiler -t python js_cannot_convert_null.ksy
js_cannot_convert_null.ksy: /instances/foo/value:
        error: expected string, got null
```

---

A footnote to the code change: I removed `_: Int` because it was
redundant - JavaScript doesn't have this type. `int` only exists in
Java, JavaScript uses `_: Double` for all numbers.

js/src/main/scala/io/kaitai/struct/format/JavaScriptKSYParser.scala

@@ -29,12 +29,20 @@ object JavaScriptKSYParser {
 
   def yamlJavascriptToScala(src: Any): Any = {
     src match {
-      case array: js.Array[AnyRef] =>
+      case array: js.Array[_] =>
         array.toList.map(yamlJavascriptToScala)
-      case _: String | _: Int | _: Double | _: Boolean =>
+      // See <https://www.scala-js.org/api/scalajs-library/1.13.1/scala/scalajs/js/index.html>:
+      //
+      // > There are no explicit definitions for JavaScript primitive types, as
+      // > one could expect, because the corresponding Scala types stand in
+      // > their stead:
+      // > * (...)
+      // > * `Unit` is the type of the JavaScript undefined value
+      // > * `Null` is the type of the JavaScript null value
+      case _: Boolean | _: Double | _: String | _: Unit | null =>
         src
       case dict =>
-        dict.asInstanceOf[js.Dictionary[AnyRef]].toMap.view.mapValues(yamlJavascriptToScala).toMap
+        dict.asInstanceOf[js.Dictionary[_]].toMap.view.mapValues(yamlJavascriptToScala).toMap
     }
   }
 }