add path replace-extension to stdlib-candidate (#1002)

Adds `path replace-extension` as requested in nushell/nushell#14144

Also sets up testing for candidates. In order to do this, I made
some changes:

1. ported `nu-std/testing.nu` under `stdlib-candidate` folder, and
making some changes.
2. run candidate tests in `toolkit check pr` command, to make sure the
test is run in CI.
3. including `stdlib-candidate` to `NU_LIB_DIRS` when running lint, so
the tests can pass linter.

Changes in stdlib-candidate/testing.nu:

1. remove `std/log` usage
2. including `stdlib-candidate` path in `run-test` command

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 +1,30 @@
+# 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 replace-extension "rs"
+# ab.rs
+# > "ab.txt" | path replace-extension ".rs"
+# ab.rs
+#
+# - setting a list of input path ext to `rs`
+# > ["ab.txt", "cd.exe"] | path replace-extension "rs"
+# ╭───┬──────────╮
+# │ 0 │ ab.rs    │
+# │ 1 │ cd.rs    │
+# ╰───┴──────────╯
+# ``` 
+export def replace-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
+    }
+}
+