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/tests/script-parsing.nu

@@ -0,0 +1,37 @@
+use std assert
+use ../std-rfc parse-arg
+
+const SPAN = { start: 0, end: 0 }
+
+export def "test parse-arg ok" [] {
+    const TEST_CASES = [
+        [ input, type, expected ];
+
+        [ "123", "int", 123 ],
+        [ "[1, 2, 3]", "list<int>", [1, 2, 3] ],
+        [ "'spam'", "string", "spam" ],
+        [
+            "{ a: 1, b: 'egg', c: false }",
+            "record<a: int, b: string, c: bool>",
+            { a: 1, b: 'egg', c: false },
+        ],
+    ]
+
+    for t in $TEST_CASES {
+        assert equal ($t.input | parse-arg $SPAN $t.type) $t.expected
+    }
+}
+
+export def "test parse-arg err" [] {
+    const TEST_CASES = [
+        [ input, type ];
+
+        [ "{ invalid NUON", "" ],
+        [ "[1, 2, 3]", "string" ],
+    ]
+
+    for t in $TEST_CASES {
+        let msg = $"test case: input: '($t.input)', type: ($t.type)"
+        assert error { $t.input | parse-arg $SPAN $t.type } $msg
+    }
+}