feat: Supports using Ruby language to extend PPE language instructions.

[snowyu]

Inline ruby function to extend instruction:

- !fn |-
    [ruby] def add(a,b)
      return a+b
    end
- $add(1,2)

Import ruby functions from file:

---
import: "./my.rb"
---
- $add(1,2)

# my.rb
def add(a,b)
  return a+b
end