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/str_xpend.nu

@@ -0,0 +1,12 @@
+use std assert
+use ../std-rfc str
+
+export def "test append" [] {
+    assert equal ("foo" | str append "/") "foo/"
+    assert equal (["foo", "bar", "baz"] | str append "/") ["foo/", "bar/", "baz/"]
+}
+
+export def "test prepend" [] {
+    assert equal ("foo" | str prepend "/") "/foo"
+    assert equal (["foo", "bar", "baz"] | str prepend "/") ["/foo", "/bar", "/baz"]
+}