Added Image constructor support

[tomvanenckevort]

Types of Changes

Prerequisites

Please make sure you can check the following two boxes:

• I have read the CONTRIBUTING document
• My code follows the code style of this project

Contribution Type

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue, please reference the issue id)
• New feature (non-breaking change which adds functionality, make sure to open an associated issue first)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Description

This PR adds support for the Image constructor as described here.

Since this constructor does not directly link to the HTMLImageElement constructor, I have added a new "constructor function" way of adding constructors to the Window object:

• Added DomConstructorFunctionInstance which stores a reference to the constructor function and calls it when Jint calls the Construct function.
• Added DomConstructorFunctionAttribute to decorate any methods that need to be treated as a constructor function.
• Added methods in the EngineExtensions, EngineInstance and CreatorCache to find any decorated constructor functions and add them to the Window object.
• Added new extension method to WindowExtensions which handles the Image constructor call and returns a new IHtmlImageElement instance.
• Added unit test to test new Image constructor.

[FlorianRappl]

LGTM - thanks! 🚀