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/platform/ansi/ansi_.rs

@@ -738,9 +738,9 @@ fn heavy_lifting(code: Value, escape: bool, osc: bool, call: &Call) -> Result<St
         });
     }
     let code_string = if param_is_string {
-        code.coerce_string().expect("error getting code as string")
+        code.coerce_str().expect("error getting code as string")
     } else {
-        "".to_string()
+        "".into()
     };
     let param_is_valid_string = param_is_string && !code_string.is_empty();
     if (escape || osc) && (param_is_valid_string) {