Allow (_ to_fp ...) in strict parsing mode (#2566)

Allow (_ to_fp ...) in strict parsing mode (#2566)

When parsing with `--strict-parsing`, we are checking whether the
operators that we encounter have been explicitly added to the
`d_logicOperators` set in the `Parser` class. We did not do that for the
indexed operator `(_ to_fp ...)` (which is represented by the kind
`FLOATINGPOINT_TO_FP_GENERIC`). This commit adds the operator.