[expressions] with_variable shouldn't allow to set a variable already reserved (e.g. @geometry, @id, @feature, ...)

[agiudiceandrea]

Feature description

The with_variable function currently allows to set e.g. a 'geometry' variable without any warning that such variable will not actually store the desired value:
with_variable('geometry', geom_from_wkt('POINT(3 1)'), geom_to_wkt(@geometry))
returns the WKT of the geometry or the current feature instead of POINT(3 1).

Additional context

No response