Introduce flat and expanded terminology

For function signatures

```r
# Flat
fn <- function(x, y, z) {
  body
}

# Expanded
fn <- function(
  x, 
  y, 
  z
) {
  body
}
```

For function calls

```r
list(x, y, z)

list(
  x, 
  y, 
  z
)
```