"short" Welcome Banner option (#14638)

# Description

Adds:

```nushell
$env.config.show_banner = "short"
```

This will display *only* the startup time. That was the only information
from the banner that the user couldn't possibly include in their own
config/banner (since it is `-1ns` during startup). This allows one to
create their own banner and yet still show the startup time.

Example (can be a file named `banner.nu` in autoloads:

```nushell
$env.config.show_banner = "short"

let ver = (version)
print $"(ansi blue_bold)Nushell Release:(ansi reset) ($ver.version) \(($ver.build_os)\)"
```


![image](https://github.com/user-attachments/assets/dd9d53a2-d89a-432e-8fa3-2d65072e08b1)


---

`true` and `false` settings continue to work as they do today. `true` is
still the default.

# User-Facing Changes

New configuration option:

```nushell
$env.config.show_banner = "short"
```

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

◼️ Update doc
◼️ Update `doc_config.nu`

crates/nu-std/std/core/mod.nu

@@ -1,11 +1,17 @@
 use std/dt [datetime-diff, pretty-print-duration]
 
 # Print a banner for nushell with information about the project
-export def banner [] {
+export def banner [
+    --short    # Only show startup time
+] {
 let dt = (datetime-diff (date now) 2019-05-10T09:59:12-07:00)
 let ver = (version)
+let startup_time = $"(ansi green_bold)Startup Time: (ansi reset)($nu.startup-time)"
 
-let banner_msg = $"(ansi green)     __  ,(ansi reset)
+let banner_msg = match $short {
+    true => $"($startup_time)(char newline)"
+
+    false => $"(ansi green)     __  ,(ansi reset)
 (ansi green) .--\(\)°'.' (ansi reset)Welcome to (ansi green)Nushell(ansi reset),
 (ansi green)'|, . ,'   (ansi reset)based on the (ansi green)nu(ansi reset) language,
 (ansi green) !_-\(_\\    (ansi reset)where all data is structured!
@@ -20,8 +26,9 @@ Learn how to remove this at: (ansi green)https://nushell.sh/book/configuration.h
 It's been this long since (ansi green)Nushell(ansi reset)'s first commit:
 (pretty-print-duration $dt)
 
-Startup Time: ($nu.startup-time)
+($startup_time)
 "
+}
 
 match $env.config?.use_ansi_coloring? {
     false => { $banner_msg | ansi strip }