Introduce else if in PoE syntax

We cannot express `else if`, leading to a somewhat verbose coding style. For example:

```
if x = 3 then ...
else
  if x = 4 then ...
  else
    if x = 5 then ...
    else
      ...
```

With `else if` or `elif`, we can express the code above into this one:

```
if x = 3 then ...
elif x = 4 then ...
elif x = 5 then ...
elif ...
```