Rename std/core to std/prelude (#14962)

`std/core` is always loaded by Nushell during startup, and the 
commands in it are always available. As such, it's renamed
`std/prelude`.

`scope modules` and `view files` now show `prelude` in place of
`core`.

crates/nu-std/std/prelude/mod.nu

@@ -0,0 +1,48 @@
+use std/dt [datetime-diff, pretty-print-duration]
+
+# Print a banner for nushell with information about the project
+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 = 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!
+
+Version: (ansi green)($ver.version) \(($ver.build_os)\)
+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)
+And the (ansi green)Latest Nushell News(ansi reset) at (ansi green)https://nushell.sh/blog/(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)
+"
+}
+
+match (config use-colors) {
+    false => { $banner_msg | ansi strip }
+    _ => $banner_msg
+}
+}
+
+# Return the current working directory
+export def pwd [
+    --physical (-P) # resolve symbolic links
+] {
+    if $physical {
+        $env.PWD | path expand
+    } else {
+        $env.PWD
+    }
+}