isa keyword is still used incorrrectly

[JSAbrahams]

Description of Bug

isa should only be used in boolean expressions. In previous incarnation of grammar, it was also used in class descriptions to show a class extends another. However, we now use : that, but I still see it in the tests.

For instnace, in generics.mamba:

class Err1(msg: String) isa Exception(msg)
class Err2(msg: String) isa Exception(msg)

Expected behavior

It should be

class Err1(msg: String): Exception(msg)
class Err2(msg: String): Exception(msg)

[JSAbrahams]

This was already resolved at some point.