Core: Fix recursion level check for VariantWriter::write() with objects

[dalexeev]

Fixes a crash on the following code:

class Test extends Resource:
    @export var a: Resource

func _run() -> void:
    var test := Test.new()
    test.a = test
    print(var_to_str(test))

See #81114 (comment).

Also includes minor code style changes (recursion_count → p_recursion_count, blank lines).

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.2.