Add test support crate for plugin developers (#12259)

# Description

Adds a `nu-plugin-test-support` crate with an interface that supports
testing plugins.

Unlike in reality, these plugins run in the same process on separate
threads. This will allow
testing aspects of the plugin internal state and handling serialized
plugin custom values easily.
We still serialize their custom values and all of the engine to plugin
logic is still in play, so
from a logical perspective this should still expose any bugs that would
have been caused by that.
The only difference is that it doesn't run in a different process, and
doesn't try to serialize
everything to the final wire format for stdin/stdout.

TODO still:

- [x] Clean up warnings about private types exposed in trait definition
- [x] Automatically deserialize plugin custom values in the result so
they can be inspected
- [x] Automatic plugin examples test function
- [x] Write a bit more documentation
- [x] More tests
- [x] Add MIT License file to new crate

# User-Facing Changes

Plugin developers get a nice way to test their plugins.

# Tests + Formatting
Run the tests with `cargo test -p nu-plugin-test-support --
--show-output` to see some examples of what the failing test output for
examples can look like. I used the `difference` crate (MIT licensed) to
make it look nice.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

- [ ] Add a section to the book about testing
- [ ] Test some of the example plugins this way
- [ ] Add example tests to nu_plugin_template so plugin developers have
something to start with

crates/nu-protocol/src/value/mod.rs

@@ -754,9 +754,9 @@ impl Value {
         }
     }
 
-    /// Update the value with a new span
-    pub fn with_span(mut self, new_span: Span) -> Value {
-        match &mut self {
+    /// Set the value's span to a new span
+    pub fn set_span(&mut self, new_span: Span) {
+        match self {
             Value::Bool { internal_span, .. }
             | Value::Int { internal_span, .. }
             | Value::Float { internal_span, .. }
@@ -777,7 +777,11 @@ impl Value {
             | Value::CustomValue { internal_span, .. } => *internal_span = new_span,
             Value::Error { .. } => (),
         }
+    }
 
+    /// Update the value with a new span
+    pub fn with_span(mut self, new_span: Span) -> Value {
+        self.set_span(new_span);
         self
     }