Add custom datetime format through strftime strings (#9500)

- improves usability of datetime's in displayed text
- 
# Description
Creates a config point for specifying long / short date time formats.
Defaults to humanized as we have today.

Provides for adding strftime formats into config.nu such as:
```nu
  datetime_format: {
    normal: "%Y-%m-%d %H:%M:%S"
    table: "%Y-%m-%d"
  }
```

Example:
```bash
> $env.config.datetime_format                                                                                                                         
┏━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ normal ┃ %a, %d %b %Y %H:%M:%S %z ┃
┃ table  ┃ %m/%d/%y %I:%M:%S%p      ┃
┗━━━━━━━━┻━━━━━━━━━━━━━━━━━━━━━━━━━━┛
> let a = (date now)                                                                                                                                  
> echo $a                                                                                                                                             
Thu, 22 Jun 2023 10:21:23 -0700
> echo [$a]                                                                                                                                           
┏━━━┳━━━━━━━━━━━━━━━━━━━━━┓
┃ 0 ┃ 06/22/23 10:21:23AM ┃
┗━━━┻━━━━━━━━━━━━━━━━━━━━━┛
```

# User-Facing Changes
Any place converting a datetime to a user displayed value should be
impacted.

# Tests + Formatting

- `cargo fmt --all -- --check` Done
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect -A clippy::result_large_err` Done
- `cargo test --workspace` Done 
- `cargo run -- crates/nu-std/tests/run.nu` Not done - doesn't seem to
work

```bash
> use toolkit.nu  # or use an `env_change` hook to activate it automatically
> toolkit check pr
``` - Done

---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>
Co-authored-by: Antoine Stevan <44101798+amtoine@users.noreply.github.com>

crates/nu-utils/src/sample_config/default_config.nu

@@ -207,6 +207,15 @@ let-env config = {
     }
   }
 
+  # datetime_format determines what a datetime rendered in the shell would look like.
+  # Behavior without this configuration point will be to "humanize" the datetime display,
+  # showing something like "a day ago."
+
+  datetime_format: {
+    normal: '%a, %d %b %Y %H:%M:%S %z'  # shows up in displays of variables or other datetime's outside of tables
+    # table: '%m/%d/%y %I:%M:%S%p'        # generally shows up in tabular outputs such as ls. commenting this out will change it to the default human readable datetime format
+  }
+
   explore: {
     help_banner: true
     exit_esc: true