Parsing for nested if statements

Fortunately the Engine already handles this, thanks to the structure of elseif statements.

Whitespace / indentation of nested statements has not yet been addressed. Should come in a later PR, this feature is primarily to allow if statements nested inside snippets.

https://www.agileplannerapp.com/boards/105200/cards/8756
Doubling checking, but will this handle?
```
$IF pred1?
#{first_true_body}
$IF pred2?
#{both_true_body}
$ENDIF
#{first_true_body}
$ENDIF
```