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/tests/test_prelude.nu

@@ -0,0 +1,7 @@
+use std/assert
+
+#[test]
+def banner [] {
+    use std/prelude
+    assert ((prelude banner | lines | length) == 16)
+}