Multi-line REPL

First step towards closing #239 (closed). Incorporates the proposed changes to cpp-terminal from PR 21 into LFortran. As it stands, a new line can be forced with Alt + Enter, and an issue with the REPL hanging when the input string length exceeds the terminal line length is also solved. The achieve this, the terminal detection logic was merged into the cpp-terminal PR, and some bug fixes and quality of life improvements were added to the prompt. Additional features, like automatic detection of potential multi-line statements when the user presses Enter instead of Alt + Enter, will be completed separately.