Implement Display for CellPath (#11023)

# Description
Because `CellPath::into_string` takes a borrowed `self`, I renamed it to
`to_string` to follow Rust [API
guidelines](https://rust-lang.github.io/api-guidelines/naming.html#ad-hoc-conversions-follow-as_-to_-into_-conventions-c-conv).
This then triggered the clippy lint
[inherent_to_string](https://rust-lang.github.io/rust-clippy/master/index.html#/inherent_to_string),
which is... correct! The current `CellPath::into_string` is being used
as if it were the `Display` implementation for `CellPath`.

# User-Facing Changes
Breaking API change for `nu-protocol`, since `CellPath::into_string` was
removed.

crates/nu-command/src/formats/to/text.rs

@@ -146,7 +146,7 @@ fn local_into_string(value: Value, separator: &str, config: &Config) -> String {
         Value::Nothing { .. } => String::new(),
         Value::Error { error, .. } => format!("{error:?}"),
         Value::Binary { val, .. } => format!("{val:?}"),
-        Value::CellPath { val, .. } => val.into_string(),
+        Value::CellPath { val, .. } => val.to_string(),
         Value::CustomValue { val, .. } => val.value_string(),
         Value::MatchPattern { val, .. } => format!("{:?}", val),
     }