Add `io.inputcast`

[trag1c]

⚠️ Make sure you're contributing to the 0.4.0 branch!

This function should work just like ???, but it should try to convert inputs to specific types:

• "1" → /
• "1 2" → [/, /\]
• "3, 4" → [//, /\\]
• "" → ‎ ‎ (NULL)
• "3..7" → <<//..///>>
• "a=1, b=2" → {{"a" -> /, "b" -> /\}}
• "hi" → "hi"

<-io.inputcast;

=> * {
    inputcast("Enter a number to double: ") ++ /\!;
}

Enter a number to double: 7
14

The prompt should be optional ("" by default).