test: JavaScript unit tests

Implements unit tests for the Javascript bindings based on duktape. All errors found are fixed in the same commit.

This commit also includes the complete implementation for classes that were only placeholders, including conversions to dom::Value and related wrapper classes for objects, arrays, and functions.

The changes are only related to bindings from Javascript to C++. Bindings from C++ to Javascript will be addressed in a later commit.

include/mrdox/Dom/Object.hpp

@@ -357,6 +357,14 @@ class MRDOX_DECL
     virtual void set(String key, Value value) = 0;
 
     /** Invoke the visitor for each key/value pair.
+
+        The visitor function must return `true` to
+        continue iteration, or `false` to stop.
+
+        The visit function returns `true` if the
+        visitor returned `true` for all elements,
+        otherwise `false`.
+
     */
     virtual bool visit(std::function<bool(String, Value)>) const = 0;
 

include/mrdox/Dom/Value.hpp

@@ -238,19 +238,32 @@ class MRDOX_DECL
     */
     bool isTruthy() const noexcept;
 
+    /** Return the underlying boolean value.
+
+        @note Behaviour is undefined if `!isBoolean()`
+
+     */
     bool getBool() const noexcept
     {
         MRDOX_ASSERT(isBoolean());
         return b_ != 0;
     }
 
+    /** Return the underlying integer value.
+
+        @note Behaviour is undefined if `!isInteger()`
+    */
     std::int64_t
     getInteger() const noexcept
     {
         MRDOX_ASSERT(isInteger());
         return i_;
     }
 
+    /** Return the underlying string value.
+
+        @note Behaviour is undefined if `!isString()`
+    */
     String const&
     getString() const noexcept
     {
@@ -333,6 +346,16 @@ class MRDOX_DECL
     bool
     exists(std::string_view key) const;
 
+    /** Return if an Array or Object is empty.
+    */
+    bool
+    empty() const;
+
+    /** Return if an Array or Object is empty.
+    */
+    std::size_t
+    size() const;
+
     /** Invoke the function.
 
         If the Value is not an object, or the key
@@ -349,16 +372,6 @@ class MRDOX_DECL
         return getFunction()(std::forward<Args>(args)...);
     }
 
-    /** Return if an Array or Object is empty.
-    */
-    bool
-    empty() const;
-
-    /** Return if an Array or Object is empty.
-    */
-    std::size_t
-    size() const;
-
     /// @copydoc isTruthy()
     explicit
     operator bool() const noexcept
@@ -506,7 +519,7 @@ class MRDOX_DECL
         return lhs && Value(rhs);
     }
 
-    /** Return a diagnostic string.
+    /** Return value as a string.
     */
     friend
     std::string