Merge stream_example into example plugin and clean up names (#12234)

# Description

As suggested by @WindSoilder, since plugins can now contain both simple
commands that produce `Value` and commands that produce `PipelineData`
without having to choose one or the other for the whole plugin, this
change merges `stream_example` into `example`.

# User-Facing Changes

All of the example plugins are renamed.

# Tests + Formatting

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

# After Submitting

- [ ] Check nushell/nushell.github.io for any docs that match the
command names changed

crates/nu_plugin_example/src/lib.rs

@@ -13,12 +13,21 @@ impl Plugin for Example {
         //
         // If it doesn't appear on this list, it won't be added.
         vec![
-            Box::new(NuExample1),
-            Box::new(NuExample2),
-            Box::new(NuExample3),
-            Box::new(NuExampleConfig),
-            Box::new(NuExampleEnv),
-            Box::new(NuExampleDisableGc),
+            Box::new(Main),
+            // Basic demos
+            Box::new(One),
+            Box::new(Two),
+            Box::new(Three),
+            // Engine interface demos
+            Box::new(Config),
+            Box::new(Env),
+            Box::new(DisableGc),
+            // Stream demos
+            Box::new(CollectExternal),
+            Box::new(ForEach),
+            Box::new(Generate),
+            Box::new(Seq),
+            Box::new(Sum),
         ]
     }
 }