Deprecate the nu_scripts version of stdlib-candidate (#1042)

Deprecates the existing `stdlib-candidate` directories in `nu_scripts`
since `std-rfc` has now transitioned to the main repo. Updates readme
and renamed directories.

stdlib-candidate-archive/stdlib-candidate-older/std-rfc/script-parsing.nu

@@ -0,0 +1,61 @@
+# helps parsing CLI arguments for Nushell scripts
+#
+# the following Nushell script does not make sense to be used as an external
+# command because there is no such thing as a `list<string>` in Bash for
+# instance.
+# ```nushell
+# def main [x: list<int>] {
+#     print $x
+# }
+# ```
+#
+# one needs to write something less strict at parse-time and thus looses type
+# information...
+# ```nushell
+# def main [
+#     x: string, # list<int>
+# ] {
+#     print $x
+# }
+# ```
+#
+# it's possible to write a much stronger script with `parse-arg`
+# ```nushell
+# def main [
+#     x: string, # list<int>
+# ] {
+#     let x = $x | parse-arg (metadata $x).span "list<int>" # the script would crash if either
+#                                                           # `$x: string` is not valid NUON or if
+#                                                           # the resulting value is not a `list<int>`
+#     print $x                                              # here, `$x` is a `list<int>` as intended
+# }
+# ```
+export def parse-arg [
+    span: record<start: int, end: int>, # the span of the input variable
+    expected_type: string, # the expected type for the input variable
+]: [ string -> any ] {
+    let val = try {
+        $in | from nuon
+    } catch {
+        error make {
+            msg: $"(ansi red_bold)invalid NUON(ansi reset)",
+            label: {
+                text: "invalid NUON",
+                span: $span,
+            },
+        }
+    }
+
+    if ($val | describe) != $expected_type {
+        error make {
+            msg: $"(ansi red_bold)bad type(ansi reset)",
+            label: {
+                text: $"type: ($val | describe)",
+                span: $span,
+            },
+            help: $"expected ($expected_type)",
+        }
+    }
+
+    $val
+}