Add a warning for implicit conversion in an explicitly typed declaration

[Yagich]

This code:

var foo: bool = 10.0

is valid and the value of foo is true, since it implicitly converts the float to a bool. It's pretty minor, but I just spent 20 minutes trying to figure out why my save files had booleans when I was sure they were numbers.

[dalexeev]

Related:

• Gdscript -> Assign a boolean value to a typed float produces no error. godot#26248
• Bools are implicitly cast to ints godot#40249
• bool -> (int, float) and (int, float) -> bool implicit cast removed godot#40336
• GDScript: Add IMPLICIT_CONVERSION_CAUSES_COPY warning godot#89853