arrow

ARROW-15712 [R] Add a `type` method for `Expression` objects

This would allow for more consistent syntax when extracting the type of an expression.
A block like this:
```r
if (inherits(x, "Expression")) {
class <- x$type()$ToString()
} else {
class <- type(x)$ToString()
}
```

would be simplified to

```r
class <- type(x)$ToString()
```

Closes #12447 from dragosmg/type_method_for_expression

Authored-by: Dragoș Moldovan-Grünfeld <dragos.mold@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>