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/path/mod.nu

@@ -0,0 +1,71 @@
+# Helper function for `path with` commands
+def with-field [field: string, value: string] {
+  path parse
+  | update $field $value
+  | path join
+}
+
+# Replace extension of input file paths.
+#
+# Note that it doesn't change the file name locally.
+#
+# # Example
+# - setting path ext to `rs`
+# ```nushell
+# > "ab.txt" | path with-extension "rs"
+# ab.rs
+# > "ab.txt" | path with-extension ".rs"
+# ab.rs
+#
+# - setting a list of input path ext to `rs`
+# > ["ab.txt", "cd.exe"] | path with-extension "rs"
+# ╭───┬──────────╮
+# │ 0 │ ab.rs    │
+# │ 1 │ cd.rs    │
+# ╰───┴──────────╯
+# ```
+export def with-extension [ext: string] {
+  let path = $in
+  let ext_trim = if $ext starts-with "." {
+    $ext | str substring 1..
+  } else {
+    $ext
+  }
+  $path | with-field extension $ext_trim
+}
+
+# Replace stem of input file paths.
+#
+# Note that it doesn't change the file name locally.
+#
+# # Example
+# - replace stem with "share"
+# ```nushell
+# > "/usr/bin" | path with-stem "share"
+# /usr/share
+#
+# - replace stem with "nushell"
+# > ["/home/alice/", "/home/bob/secret.txt"] | path with-stem "nushell"
+# ╭───┬───────────────────────╮
+# │ 0 │ /home/nushell         │
+# │ 1 │ /home/bob/nushell.txt │
+# ╰───┴───────────────────────╯
+# ```
+export def with-stem [stem: string] { with-field stem $stem }
+
+# Replace parent field of input file paths.
+#
+# # Example
+# - replace parent path with `/usr/share`
+# ```nushell
+# > "/etc/foobar" | path with-parent "/usr/share/"
+# /usr/share/foobar
+#
+# - replace parent path with `/root/` for all filenames in list
+# > ["/home/rose/meow", "/home/fdncred/"] | path with-parent "/root/"
+# ╭───┬───────────────╮
+# │ 0 │ /root/meow    │
+# │ 1 │ /root/fdncred │
+# ╰───┴───────────────╯
+# ```
+export def with-parent [parent: string] { with-field parent $parent }