Move Value to helpers, separate span call (#10121)

# Description

As part of the refactor to split spans off of Value, this moves to using
helper functions to create values, and using `.span()` instead of
matching span out of Value directly.

Hoping to get a few more helping hands to finish this, as there are a
lot of commands to update :)

# User-Facing Changes
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---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>
Co-authored-by: WindSoilder <windsoilder@outlook.com>

crates/nu-command/src/env/config/config_.rs

@@ -34,16 +34,16 @@ impl Command for ConfigMeta {
         call: &Call,
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
-        Ok(Value::String {
-            val: get_full_help(
+        Ok(Value::string(
+            get_full_help(
                 &ConfigMeta.signature(),
                 &ConfigMeta.examples(),
                 engine_state,
                 stack,
                 self.is_parser_keyword(),
             ),
-            span: call.head,
-        }
+            call.head,
+        )
         .into_pipeline_data())
     }
 

crates/nu-command/src/env/export_env.rs

@@ -58,9 +58,7 @@ impl Command for ExportEnv {
             Example {
                 description: "Set an environment variable",
                 example: r#"export-env { $env.SPAM = 'eggs' }"#,
-                result: Some(Value::Nothing {
-                    span: Span::test_data(),
-                }),
+                result: Some(Value::nothing(Span::test_data())),
             },
             Example {
                 description: "Set an environment variable and examine its value",