REFACTOR: move the banner from the rust source to the standard library (#8406)

Related to:
- #8311 
- #8353

# Description
with the new `$nu.startup-time` from #8353 and as mentionned in #8311,
we are now able to fully move the `nushell` banner from the `rust`
source base to the standard library.

this PR
- removes all the `rust` source code for the banner
- rewrites a perfect clone of the banner to `std.nu`, called `std
banner`
- call `std banner` from `default_config.nu`

# User-Facing Changes
see the demo: https://asciinema.org/a/566521

- no config will show the banner (e.g. `cargo run --release --
--no-config-file`)
- a custom config without the `if $env.config.show_banner` block and no
call to `std banner` would never show the banner
- a custom config with the block and `config.show_banner = true` will
show the banner
- a custom config with the block and `config.show_banner = false` will
NOT show the banner

# Tests + Formatting
a new test line has been added to `tests.nu` to check the length of the
`std banner` output.
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
```
$nothing
```

---------

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

crates/nu-std/src/lib.rs

@@ -78,6 +78,7 @@ pub fn load_standard_library(
             ("help", include_str!("../lib/help.nu")),
             ("testing", include_str!("../lib/testing.nu")),
             ("xml", include_str!("../lib/xml.nu")),
+            ("dt", include_str!("../lib/dt.nu")),
         ];
 
         // Define commands to be preloaded into the default (top level, unprefixed) namespace.