Provide a "macro-like" command that runs commands on the shell

[muse254]

I have deliberated on creating libraries to make the language more holistic and cover must-have libraries.
The best way to make this work out is to allow shell access and use what is already present; after all the language is platform agnostic and can literally run on the browser.

The proposal is to have something that looks somewhat like this:

filename is "lines.txt"

shell! {
 echo -n "This is line 1." | cat > {{filename}}
}

It is to be noted that I'm taking creative liberty to make it look like rust macros, the model works and I don't have good reason to invent different syntax.