Support redirect stderr and stdout+stderr with a pipe (#11708)

# Description
Close: #9673
Close: #8277
Close: #10944

This pr introduces the following syntax:
1. `e>|`, pipe stderr to next command. Example: `$env.FOO=bar nu
--testbin echo_env_stderr FOO e>| str length`
2. `o+e>|` and `e+o>|`, pipe both stdout and stderr to next command,
example: `$env.FOO=bar nu --testbin echo_env_mixed out-err FOO FOO e+o>|
str length`

Note: it only works for external commands. ~There is no different for
internal commands, that is, the following three commands do the same
things:~ Edit: it raises errors if we want to pipes for internal
commands
``` 
❯ ls e>| str length
Error:   × `e>|` only works with external streams
   ╭─[entry #1:1:1]
 1 │ ls e>| str length
   ·    ─┬─
   ·     ╰── `e>|` only works on external streams
   ╰────

❯ ls e+o>| str length
Error:   × `o+e>|` only works with external streams
   ╭─[entry #2:1:1]
 1 │ ls e+o>| str length
   ·    ──┬──
   ·      ╰── `o+e>|` only works on external streams
   ╰────
```

This can help us to avoid some strange issues like the following:

`$env.FOO=bar (nu --testbin echo_env_stderr FOO) e>| str length`

Which is hard to understand and hard to explain to users.

# User-Facing Changes
Nan

# Tests + Formatting
To be done

# After Submitting
Maybe update documentation about these syntax.

crates/nu-parser/src/flatten.rs

@@ -560,7 +560,9 @@ pub fn flatten_pipeline_element(
     pipeline_element: &PipelineElement,
 ) -> Vec<(Span, FlatShape)> {
     match pipeline_element {
-        PipelineElement::Expression(span, expr) => {
+        PipelineElement::Expression(span, expr)
+        | PipelineElement::ErrPipedExpression(span, expr)
+        | PipelineElement::OutErrPipedExpression(span, expr) => {
             if let Some(span) = span {
                 let mut output = vec![(*span, FlatShape::Pipe)];
                 output.append(&mut flatten_expression(working_set, expr));