Additional cast methods for `JsonValue`

As of 1.1, the `JsonValue` interface provides convenience methods to cast the value to `JsonObject` and `JsonArray`. Is there a reason to limit things to those two cases?

Currently, the approach to get a value via a pointer looks like:
```java
JsonObject obj = ...;
String target = ((JsonString) obj.getValue("/some/path/0/to/my/string")).getString();
```

It would be _slightly_ nicer for the following:
```java
JsonObject obj = ...;
String target = obj.getValue("/some/path/0/to/my/string").asJsonString().getString();
```

Similarly, methods for `asJsonNumber` and `asJsonBoolean`, perhaps with overloads that tolerate the `ValueType` of `NULL` with a default argument.

Start with the JsonValue interface and its existing asJsonObject and asJsonArray methods, then inspect the JsonString, JsonNumber, JsonBoolean, and ValueType APIs referenced by the issue. The work is done when the proposed cast methods and any NULL-default overloads are consistently defined and covered by the relevant project tests.

Written by the indexing model from the issue text.