Consolidate $ into a stronger compile-time only get_node, rather than a simple shorthand

[mieldepoche]

problems:

• I want GDScript to be as statically typed as it can.
  It provides warnings/errors for unsafe property and method usage: it can optionally error out on thing.function() if thing may not have a method called function at runtime. That's nice! But... it breaks the current common way of dealing with components in godot: $AnimationPlayer.play("run") make this error show up since $AnimationPlayer gets a Node, which may not have a play method at runtime...
• if I get a node $MyNode and then rename the node in my scene, I don't get an error until runtime. Ideally the script would be updated to accomodate for node renaming, but currently you have to press play to be reminded that you didn't rename your node in code.

in 100% of the cases, getting a node with $MyNode is done with a node that will not be renamed, replaced or deleted willingly at runtime.
That is, in 100% of the cases:

• the type of that node is known at compile time,
• the fact that this node has to be there is known at compile time.

solutions proposals:

• $MyNode should provide actual type information, matching the node in question.
  • Currently, you get autocompletion stuff in the editor, but the infered type is still Node. So I'm not allowed to write $AnimationPlayer.play("...") even though I get:
    • autocompletion for the method
    • autocompletion for the list of animations
  • But this has no reason of failing! Like, you'd have to be working against godot willingly for $AnimationPlayer to return something else than the animation player you put in your scene! At worst it's not there anymore and you runtime fail (and get null), but like... you don't replace that with a MeshInstance3D that you rename "AnimationPlayer" at runtime in any sane project ever made, right?
• $MyNode should complain if no node called "MyNode" is there in the scene the script is attached to.
  • Because I think that literally every time you access a node with $, it's because the node is in the scene you're editing.
• when editing a script not attached to anything in the opened scene, it would behave like it does currently. But I don't believe anyone is using $MyNode to get something else than a node in the currently edited scene. (that is, you don't use $MyNode on a generic script you'll attach to arbitrary nodes. For that you either export a variable, use get_node_or_null or even get_node, but not $.)

[theraot]

There are bugs in autocompletion that needs to be addressed. In particular, getting the node on a typed readonly variable should be enough to have full autocompletion (but currently it isn't).

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I would like to have a warning (that could be configured to an error via settings) in the editor, if an script attached to a node uses either $ or % syntax but the node cannot be found. I'm not suggesting to make it an error by default simply because it could be a node added in runtime, I would probably not use that syntax in that situation, but I don't know if there is people who do.

And, of course, the warning could not be shown if the script is being edited in a context where no node having it attached is available.

Since an script could be attached to multiple nodes, or none, I'm thinking this might not be an script warning, but a node warning, or both (so that the editor can point in the scene dock which node have the problem). Sadly there is no way to jump from a node warning to a line of code, which is why I suggest it might be both, however, perhaps a way to allow such jump could be devised.

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I would also like an option in the Godot editor that would go over every scene and every script and collect error/warnings, so that problems with parts of the code that do not execute often do not have to wait for runtime to be discovered, and me opening each scene and script manually to check weren't a thing.

[Calinou]

• Related to Make the $ operator in GDScript cache node references (or add a $$ operator) #996.