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/lib/mod.nu

@@ -9,6 +9,7 @@ export use iter *
 export use log *
 export use testing *
 export use xml *
+export use dt [datetime-diff, pretty-print-duration]
 
 # Add the given paths to the PATH.
 #
@@ -171,7 +172,7 @@ def "from ns" [] {
 # >
 # ```
 #
-# > **Note**  
+# > **Note**
 # > `std bench --pretty` will return a `string`.
 #
 # # Examples
@@ -224,3 +225,27 @@ export def bench [
         $report
     }
 }
+
+# print a banner for nushell, with information about the project
+#
+# Example:
+# an example can be found in [this asciinema recording](https://asciinema.org/a/566513)
+export def banner [] {
+let dt = (datetime-diff (date now) 2019-05-10T09:59:12-07:00)
+$"(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!
+
+Please join our (ansi purple)Discord(ansi reset) community at (ansi purple)https://discord.gg/NtAbbGn(ansi reset)
+Our (ansi green_bold)GitHub(ansi reset) repository is at (ansi green_bold)https://github.com/nushell/nushell(ansi reset)
+Our (ansi green)Documentation(ansi reset) is located at (ansi green)https://nushell.sh(ansi reset)
+(ansi cyan)Tweet(ansi reset) us at (ansi cyan_bold)@nu_shell(ansi reset)
+Learn how to remove this at: (ansi green)https://nushell.sh/book/configuration.html#remove-welcome-message(ansi reset)
+
+It's been this long since (ansi green)Nushell(ansi reset)'s first commit:
+(pretty-print-duration $dt)
+
+Startup Time: ($nu.startup-time)
+"
+}