"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-protocol/src/config/mod.rs

@@ -59,7 +59,7 @@ pub struct Config {
     pub rm: RmConfig,
     pub shell_integration: ShellIntegrationConfig,
     pub buffer_editor: Value,
-    pub show_banner: bool,
+    pub show_banner: Value,
     pub bracketed_paste: bool,
     pub render_right_prompt_on_last_line: bool,
     pub explore: HashMap<String, Value>,
@@ -82,7 +82,7 @@ pub struct Config {
 impl Default for Config {
     fn default() -> Config {
         Config {
-            show_banner: true,
+            show_banner: Value::bool(true, Span::unknown()),
 
             table: TableConfig::default(),
             rm: RmConfig::default(),