add support for bracket index access to `list` in `jlox`

[zxul767]

currently, access is achieved via:

var l = list()
l.append(1)
print l.at(0)

but given how common such an operation is, and that list is a primitive, we should support the following syntax as well:

print l[0]

where the index can, of course, be any valid expression that evaluates to an integer.

[zxul767]

this is now in main.