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-cli/src/repl.rs

@@ -638,9 +638,7 @@ fn do_auto_cd(
 
     let shells = stack.get_env_var(engine_state, "NUSHELL_SHELLS");
     let mut shells = if let Some(v) = shells {
-        v.as_list()
-            .map(|x| x.to_vec())
-            .unwrap_or_else(|_| vec![cwd])
+        v.into_list().unwrap_or_else(|_| vec![cwd])
     } else {
         vec![cwd]
     };
@@ -707,7 +705,7 @@ fn do_run_cmd(
 
     if shell_integration {
         if let Some(cwd) = stack.get_env_var(engine_state, "PWD") {
-            let path = cwd.as_string()?;
+            let path = cwd.coerce_into_string()?;
 
             // Try to abbreviate string for windows title
             let maybe_abbrev_path = if let Some(p) = nu_path::home_dir() {
@@ -746,7 +744,7 @@ fn do_shell_integration_finalize_command(
 ) -> Result<()> {
     run_ansi_sequence(&get_command_finished_marker(stack, engine_state))?;
     if let Some(cwd) = stack.get_env_var(engine_state, "PWD") {
-        let path = cwd.as_string()?;
+        let path = cwd.coerce_into_string()?;
 
         // Supported escape sequences of Microsoft's Visual Studio Code (vscode)
         // https://code.visualstudio.com/docs/terminal/shell-integration#_supported-escape-sequences