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/README.md

@@ -29,6 +29,6 @@ $env.config = {
 To list plugin values run:
 
 ```nushell
-nu-example-config
+example config
 ```