Move std-rfc into Nushell (#15042)

Move `std-rfc` into Nushell.  `use std-rfc/<submodule>` now works "out-of-the-box"

crates/nu-std/std-rfc/mod.nu

@@ -0,0 +1,9 @@
+export use conversions *
+export use tables *
+export use path *
+export module clip
+export module str
+
+# kv module depends on sqlite feature, which may not be available in some builds
+const kv_module = if ("sqlite" in (version).features) { "std-rfc/kv" } else { null }
+export use $kv_module *