[Feature] compile time macros

i think we should allow for some macro-like syntax that is evaluated at compile time to save runtime.

not sure how to implement this, though. i think that they should emit amber code that is inserted into the places where they are called. like this:

```ab
macro compile_env(var: Text): Text {
return text_value(trust $\${var}$);
}

echo compile_env("LANG");
```

which will evaluate to something like this, assuming that `LANG` is `en` on compile time:

```ab
echo "en"
```

`text_value()` returned a text literal with the value of `trust $\${var}$`, which was evaluated on compile time

---

ref #436 , but this issue is for discussing the implementation of macros themselves, not the `compile_env!()` macro specifically