Add export-env command (#6355)

* WIP Start export-env

* Add missing file

* Do not modify the parser

Let's leave that for later

* Enable tests for export-env; Fmt

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

@@ -0,0 +1,74 @@
+use nu_engine::{eval_block, redirect_env, CallExt};
+use nu_protocol::{
+    ast::Call,
+    engine::{CaptureBlock, Command, EngineState, Stack},
+    Category, Example, PipelineData, Signature, Span, SyntaxShape, Value,
+};
+
+#[derive(Clone)]
+pub struct ExportEnv;
+
+impl Command for ExportEnv {
+    fn name(&self) -> &str {
+        "export-env"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("export-env")
+            .required(
+                "block",
+                SyntaxShape::Block(Some(vec![])),
+                "the block to run to set the environment",
+            )
+            .category(Category::Env)
+    }
+
+    fn usage(&self) -> &str {
+        "Run a block and preserve its environment in a current scope."
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        caller_stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+        let capture_block: CaptureBlock = call.req(engine_state, caller_stack, 0)?;
+        let block = engine_state.get_block(capture_block.block_id);
+        let mut callee_stack = caller_stack.captures_to_stack(&capture_block.captures);
+
+        let _ = eval_block(
+            engine_state,
+            &mut callee_stack,
+            block,
+            input,
+            call.redirect_stdout,
+            call.redirect_stderr,
+        );
+
+        redirect_env(engine_state, caller_stack, &callee_stack);
+
+        Ok(PipelineData::new(call.head))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Set an environment",
+            example: r#"export-env { let-env SPAM = 'eggs' }; $env.SPAM"#,
+            result: Some(Value::string("eggs", Span::test_data())),
+        }]
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(ExportEnv {})
+    }
+}