Allow multiline conditionals

[stpierre]

Terraform allows a ternary to be split among multiple lines, with comments interspersed. For instance:

count = (
  var.tgw_name == "" ?  # put a comment here for fun
  0 :
  var.number_of_az
)

Previously, the grammar only supported single-line ternary expressions.

[aoskotsky-amplify]

Nice catch @stpierre. I'll try to get this merged soon. Thanks!