Name the Value conversion functions more clearly (#11851)

# Description
This PR renames the conversion functions on `Value` to be more consistent.
It follows the Rust [API guidelines](https://rust-lang.github.io/api-guidelines/naming.html#ad-hoc-conversions-follow-as_-to_-into_-conventions-c-conv) for ad-hoc conversions.
The conversion functions on `Value` now come in a few forms:
- `coerce_{type}` takes a `&Value` and attempts to convert the value to
`type` (e.g., `i64` are converted to `f64`). This is the old behavior of
some of the `as_{type}` functions -- these functions have simply been
renamed to better reflect what they do.
- The new `as_{type}` functions take a `&Value` and returns an `Ok`
result only if the value is of `type` (no conversion is attempted). The
returned value will be borrowed if `type` is non-`Copy`, otherwise an
owned value is returned.
- `into_{type}` exists for non-`Copy` types, but otherwise does not
attempt conversion just like `as_type`. It takes an owned `Value` and
always returns an owned result.
- `coerce_into_{type}` has the same relationship with `coerce_{type}` as
`into_{type}` does with `as_{type}`.
- `to_{kind}_string`: conversion to different string formats (debug,
abbreviated, etc.). Only two of the old string conversion functions were
removed, the rest have been renamed only.
- `to_{type}`: other conversion functions. Currently, only `to_path`
exists. (And `to_string` through `Display`.)

This table summaries the above:
| Form | Cost | Input Ownership | Output Ownership | Converts `Value`
case/`type` |
| ---------------------------- | ----- | --------------- |
---------------- | -------- |
| `as_{type}` | Cheap | Borrowed | Borrowed/Owned | No |
| `into_{type}` | Cheap | Owned | Owned | No |
| `coerce_{type}` | Cheap | Borrowed | Borrowed/Owned | Yes |
| `coerce_into_{type}` | Cheap | Owned | Owned | Yes |
| `to_{kind}_string` | Expensive | Borrowed | Owned | Yes |
| `to_{type}` | Expensive | Borrowed | Owned | Yes |

# User-Facing Changes
Breaking API change for `Value` in `nu-protocol` which is exposed as
part of the plugin API.

crates/nu_plugin_query/src/query_json.rs

@@ -8,7 +8,7 @@ pub fn execute_json_query(
     input: &Value,
     query: Option<Spanned<String>>,
 ) -> Result<Value, LabeledError> {
-    let input_string = match &input.as_string() {
+    let input_string = match &input.coerce_string() {
         Ok(s) => s.clone(),
         Err(e) => {
             return Err(LabeledError {

crates/nu_plugin_query/src/query_web.rs

@@ -318,7 +318,7 @@ mod tests {
             Span::test_data(),
         );
         let config = nu_protocol::Config::default();
-        let out = item.into_string("\n", &config);
+        let out = item.to_expanded_string("\n", &config);
         assert_eq!("[[Coffee]]".to_string(), out)
     }
 
@@ -340,7 +340,7 @@ mod tests {
                     .as_list()
                     .unwrap()
                     .iter()
-                    .map(|text_nodes| text_nodes.as_string().unwrap())
+                    .map(|text_nodes| text_nodes.coerce_string().unwrap())
                     .collect::<Vec<String>>()
             })
             .collect::<Vec<Vec<String>>>();

crates/nu_plugin_query/src/query_xml.rs

@@ -21,7 +21,7 @@ pub fn execute_xpath_query(
     };
 
     let xpath = build_xpath(query_string, span)?;
-    let input_string = input.as_string()?;
+    let input_string = input.coerce_string()?;
     let package = parser::parse(&input_string);
 
     if package.is_err() {