Add FILE_PWD environment variable when running 'nu script.nu' (#7424)

# Description

When running `nu script.nu`, the `$env.FILE_PWD` will be set to the
directory where the script is.

Also makes the error message a bit nicer:
```
> target/debug/nu asdihga
Error: nu:🐚:file_not_found (link)

  × File not found
   ╭─[source:1:1]
 1 │ nu
   · ▲
   · ╰── Could not access file 'asdihga': "No such file or directory (os error 2)"
   ╰────

```

# User-Facing Changes

`FILE_PWD` environment variable is available when running a script as
`nu script.nu`.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-protocol/src/shell_error.rs

@@ -749,6 +749,15 @@ Either make sure {0} is a string, or add a 'to_string' entry for it in ENV_CONVE
     #[diagnostic(code(nu::parser::non_utf8), url(docsrs))]
     NonUtf8(#[label = "non-UTF8 string"] Span),
 
+    /// The given input must be valid UTF-8 for further processing.
+    ///
+    /// ## Resolution
+    ///
+    /// Check your input's encoding. Are there any funny characters/bytes?
+    #[error("Non-UTF8 string")]
+    #[diagnostic(code(nu::parser::non_utf8_custom), url(docsrs))]
+    NonUtf8Custom(String, #[label = "{0}"] Span),
+
     /// A custom value could not be converted to a Dataframe.
     ///
     /// ## Resolution