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/set-env.nu

@@ -0,0 +1,37 @@
+# Gracefully set an environment variable or merge a nested option.
+#
+# Examples:
+#   Set $env.NUPM_HOME
+#   > set-env NUPM_HOME $'($nu.home-path)/.local/share/nupm'
+#
+#   Add to $env.NU_LIB_DIRS
+#   > set-env --append NU_LIB_DIRS $'($env.NUPM_HOME)/modules'
+#
+#   Set a nested config option
+#   > set-env config.filesize.metric true
+#
+#   Add a config hook
+#   > set-env -a config.hooks.pre_prompt 'ellie | print'
+export def --env main [
+    field: cell-path # The environment variable name or nested option cell path
+    value: any # The value to set or append
+    --append (-a) # Append to the previous value or wrap in a new list
+]: nothing -> nothing {
+    def 'get or' [default field] {
+        get --ignore-errors $field | default $default
+    }
+    let value = if $append {
+        $env | get or [] $field | append $value
+    } else {
+        $value
+    }
+    let field = $field | to text | split row .
+    let value = match $field {
+        [_] => $value
+        [$root, ..$field] => {
+            let field = $field | into cell-path
+            $env | get or {} $root | upsert $field $value
+        }
+    }
+    load-env { ($field | first): $value }
+}