Equivalent of `expr_text()`

and `expr_label()`

for formulas.

## Examples

```
f <- ~ a + b + bc
f_text(f)
#> [1] "a + b + bc"
f_label(f)
#> [1] "`a + b + bc`"
# Names a quoted with ``
f_label(~ x)
#> [1] "`x`"
# Strings are encoded
f_label(~ "a\nb")
#> [1] "\"a\\nb\""
# Long expressions are collapsed
f_label(~ foo({
1 + 2
print(x)
}))
#> [1] "`foo(...)`"
```