Add function to define span

[meatballs]

Relating to #78

[WhiteBlackGoose]

Hey, could you explain a bit in more detail what this PR does? I've looked up the issue but still not sure what it does

[meatballs]

It adds the function MagmaDefineCell which takes a start and end position as arguments and adds a Span instance into the buffer so that the defined range becomes an executable cell.

This is so an external tool (my neovim plugin) can define cells automatically without having to rely on the cursor position each time.

[WhiteBlackGoose]

Thanks!