GDScript: Allow trying to reload shallow script when instantiated

[rune-scape]

fixes #95789

this allows automatically compiling scripts on demand when calling script.new() in a very limited way to avoid errors from the wrong order of compiling dependencies that would otherwise be more difficult to solve

i looked back at the solution i mentioned in #95789 (comment)
and i think the solution i mentioned is actually fine, the gdscript compiler is not designed for multithreading, so, without accounting for that the logic is simple:
when a script is created it has a bool shallow set to true
while reloading, reloading is set to true like usual
at the beginning of compilation, shallow is set to false
then when instantiating a not-valid shallow root script that is not currently reloading, tries to reload it

also adds a better error message and call error