Box ShellError in Value::Error (#8375)

# Description

Our `ShellError` at the moment has a `std::mem::size_of<ShellError>` of
136 bytes (on AMD64). As a result `Value` directly storing the struct
also required 136 bytes (thanks to alignment requirements).

This change stores the `Value::Error` `ShellError` on the heap.

Pro:
- Value now needs just 80 bytes
- Should be 1 cacheline less (still at least 2 cachelines)

Con:
- More small heap allocations when dealing with `Value::Error`
  - More heap fragmentation
  - Potential for additional required memcopies

# Further code changes

Includes a small refactor of `try` due to a type mismatch in its large
match.

# User-Facing Changes

None for regular users.

Plugin authors may have to update their matches on `Value` if they use
`nu-protocol`

Needs benchmarking to see if there is a benefit in real world workloads.
**Update** small improvements in runtime for workloads with high volume
of values. Significant reduction in maximum resident set size, when many
values are held in memory.

# Tests + Formatting

crates/nu-command/src/system/run_external.rs

@@ -445,7 +445,7 @@ impl ExternalCommand {
                                         ))
                                     );
                                     let _ = exit_code_tx.send(Value::Error {
-                                        error: ShellError::ExternalCommand { label: "core dumped".to_string(), help: format!("{cause}: child process '{commandname}' core dumped"), span: head },
+                                        error: Box::new(ShellError::ExternalCommand { label: "core dumped".to_string(), help: format!("{cause}: child process '{commandname}' core dumped"), span: head }),
                                     });
                                     return Ok(());
                                 }