Add Value::coerce_str (#11885)

# Description
Following #11851, this PR adds one final conversion function for
`Value`. `Value::coerce_str` takes a `&Value` and converts it to a
`Cow<str>`, creating an owned `String` for types that needed converting.
Otherwise, it returns a borrowed `str` for `String` and `Binary`
`Value`s which avoids a clone/allocation. Where possible, `coerce_str`
and `coerce_into_string` should be used instead of `coerce_string`,
since `coerce_string` always allocates a new `String`.

crates/nu-command/src/sort_utils.rs

@@ -150,7 +150,7 @@ pub fn sort(
                             .unwrap_or(Ordering::Equal)
                     }
                 } else if natural {
-                    match (a.coerce_string(), b.coerce_string()) {
+                    match (a.coerce_str(), b.coerce_str()) {
                         (Ok(left), Ok(right)) => compare_str(left, right),
                         _ => Ordering::Equal,
                     }