readonly property can be written via unqualified access

It seems that readonly properties can be written via unqualified access.

See attached project: it writes 20 to the readonly property and prints 20 when run normally, but prints a TypeError when you run it with QV4_FORCE_INTERPRETER=1.

While the compiler is happy to modify the readonly with `someNumber = 20`, it does print a TypeError if you do the write with a qualified access `root.someNumber = 20` instead.

Note: the same happens with qmllint too: it is happy with `someNumber = 20`, but complains with the qualified access `root.someNumber = 20` about `Cannot assign to read-only property someNumber [read-only-property]`.