Move std-rfc into Nushell (#15042)

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

crates/nu-std/tests/test_std-rfc_path.nu

@@ -0,0 +1,38 @@
+use std-rfc/path
+use std/assert
+
+#[test]
+def path_with_extension [] {
+    let new_path = "ab.txt" | path with-extension "rs"
+    assert equal $new_path "ab.rs"
+
+    let new_path = "ab.txt" | path with-extension ".rs"
+    assert equal $new_path "ab.rs"
+}
+
+#[test]
+def path_with_extension_for_list [] {
+    let new_path = ["ab.txt", "cd.exe"] | path with-extension "rs"
+    assert equal $new_path ["ab.rs", "cd.rs"]
+
+    let new_path = ["ab.txt", "cd.exe"] | path with-extension ".rs"
+    assert equal $new_path ["ab.rs", "cd.rs"]
+}
+
+#[test]
+def path_with_stem [] {
+    let new_path = $"(char psep)usr(char psep)bin" | path with-stem "share"
+    assert equal $new_path $"(char psep)usr(char psep)share"
+
+    let new_path = [$"(char psep)home(char psep)alice(char psep)", $"(char psep)home(char psep)bob(char psep)secret.txt"] | path with-stem "nushell"
+    assert equal $new_path [$"(char psep)home(char psep)nushell", $"(char psep)home(char psep)bob(char psep)nushell.txt"]
+}
+
+#[test]
+def path_with_parent [] {
+    let new_path = $"(char psep)etc(char psep)foobar" | path with-parent $"(char psep)usr(char psep)share(char psep)"
+    assert equal $new_path $"(char psep)usr(char psep)share(char psep)foobar"
+
+    let new_path = [$"(char psep)home(char psep)rose(char psep)meow", $"(char psep)home(char psep)fdncred(char psep)"] | path with-parent $"(char psep)root(char psep)"
+    assert equal $new_path [$"(char psep)root(char psep)meow", $"(char psep)root(char psep)fdncred"]
+}