C#: Add "Use actual type" context action for generically-typed Node::GetNode calls to resolve broken custom references

[knightofiam]

@van800 Thank you for requesting this on Discord in response to my issue.

As a long time developer but relatively new to Godot, this would have saved me hours of pain.

Pseudocode example:

Before:

Player.cs (Attached via Godot editor to a KinematicBody2D Node named 'Player'):

public class Player : KinematicBody2D
{
  ...

  // My custom method
  public void Attack (Enemy enemy)
  {
    // Implement my uber-cool attack!
  }
}

Combat.cs (Attached via Godot editor to some sibling node):

...

// In some collision method:

var player = GetNode <KinematicBody2D> ("../Player"); // Incorrect: Obtain reference to
                                                      // Player class Node instance
                                                      // which extends KinematicBody2D

player.Attack (enemy); // Error: Cannot resolve symbol 'Attack'
                       // Huh? How do I fix this?
                       // Everything "just works" in GDScript... >:(
...

In the above example, the type KinematicBody2D should be hint-underlined, whereby ALT + Enter should show a context action such as Use actual type, in order to fix broken references to fields and methods after a mis-typed call to Node::GetNode.

Additionally, if practical, add a context action to the broken method call itself, such as Fix reference by using actual type.

After:

Combat.cs (Attached via Godot editor to some sibling node):

...

// In some collision method:

var player = GetNode <Player> ("../Player"); // Correct: Obtain reference to
                                             // Player class Node instance
                                             // which extends KinematicBody2D

player.Attack (enemy); // Error resolved; FINISH HIM!!!

...

[knightofiam]

Probably related implementation-wise to #4 & #39.