Reapply "add path extension func to stdlib-candidate"

This reverts commit 43d1224.

Author: WindSoilder

stdlib-candidate/nupm.nuon

@@ -3,6 +3,6 @@
     description: "Official candidates for Nushell standard library"
     documentation: "https://github.com/nushell/nu_scripts/blob/main/stdlib-candidate/std-rfc/README.md"
     license: "https://github.com/nushell/nu_scripts/blob/main/LICENSE"
-    version: 0.4.0
+    version: 0.4.1
     type: "module"
 }

stdlib-candidate/path/mod.nu

@@ -0,0 +1,28 @@
+# Update extension of input files.
+#
+# # Example
+# - setting path ext to `rs`
+# ```nushell
+# > "ab.txt" | path extension "rs"
+# ab.rs
+# > "ab.txt" | path extension ".rs"
+# ab.rs
+#
+# - setting a list of input path ext to `rs`
+# > ["ab.txt", "cd.exe"] | path extension "rs"
+# ╭───┬──────────╮
+# │ 0 │ ab.rs    │
+# │ 1 │ cd.rs    │
+# ╰───┴──────────╯
+# ``` 
+export def extension [
+    ext: string
+] {
+    let path_parsed = $in | path parse
+    if ($ext | str starts-with ".") {
+        $path_parsed | update extension ($ext | str substring 1..) | path join
+    } else {
+        $path_parsed | update extension $ext | path join
+    }
+}
+