Support multi-line statements in the REPL

The REPL currently does not support multi-line statements. Incomplete statements will always return an error presently.

To solve this, this PR, which adds multi-line support to the cpp-terminal application used by LFortran for the REPL, needs completed. We will also need to implement some logic to catch errors, analyze tokens, etc. to determine that the user is typing a multi-line statement.