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

tests/plugins/config.rs

@@ -15,7 +15,7 @@ fn closure() {
                     }
                 }
             }
-            nu-example-config
+            example config
         "#
     );
 
@@ -27,7 +27,7 @@ fn none() {
     let actual = nu_with_plugins!(
         cwd: "tests",
         plugin: ("nu_plugin_example"),
-        "nu-example-config"
+        "example config"
     );
 
     assert!(actual.err.contains("No config sent"));
@@ -47,7 +47,7 @@ fn record() {
                     }
                 }
             }
-            nu-example-config
+            example config
         "#
     );