Change BasicREPL struct to match LineEditREPL

[non-Jedi]

I was trying to figure out what functionality from LineEditREPL
ASTInterpreter2 was using that kept it from being able to run on BasicREPL.
The stack trace it raises says:

ERROR: type BasicREPL has no field t

which is true. However BasicREPL does have an attribute terminal which is of
the same type as the attribute t of LineEditREPL. I couldn't figure out why
this might be intentional, so I thought it might be an implementation bug. For
reference, BasicREPL was added
here.
At that point, LineEditREPL already had attribute, t, and neither
BasicREPL nor LineEditREPL has changed since.

Disclaimer: I know shockingly little about how shells/terminals work, and
getting ASTInterpreter2 to run in BasicREPL could well be a fools errand,
but this seemed worth addressing anyway. If nobody can think of a reason for
LineEditREPL and BasicREPL to have the same type of attribute with different
names, I'll make a PR to fix that up.