Make onready variables created from dropping nodes include custom types

[theraot]

This pull request makes dropping a node into the script editor while holding Ctrl generate code of the form "@onready var name := $Path as Custom" "@onready var name:Custom = $Path"

Closes godotengine/godot-proposals#7239

[dalexeev]

Personally, I prefer the option

@onready var name: Type = $Path

because the following option

@onready var name := $Path as Type

hides an error if the node has the wrong type. The object will be silently cast to null, without any error.

[theraot]

@dalexeev One one hand, that makes me think that the @onready var name: Type = $Path should be the one that makes the editor happy highlights the line number. But that is a separate issue.

I'll switch the syntax based on the pull release based on the feedback.

[KoBeWi]

Last style fix needed, otherwise seems fine.

[YuriSizov]

Thanks!