Introduce explicit rails server handler option

I mistype `rails server production` instead of `rails server -e
production` expecting to lunch a server in the production environment
all the time. However, the signature of `rails server --help` is:

```
Usage:
  rails server [puma, thin etc] [options]
```

This means that the `production` argument is being interpreted as a Rack
server handler like Puma, Thin or Unicorn.

Should we argue for the `rails server production`? I'm not sure of the
reasons, but the `rails console production` behavior was deprecated in:
https://github.com/rails/rails/pull/29358, so parity with the existing
`rails console production` usage may not hold anymore.

In any case, this PR introduces an explicit option for the Rack servers
configuration. The option is called `--using` (or `-u` for short) to
avoid the `rails server --server` tantrum.

The new interface of `rails server` is:

```
Usage:
  rails server [using] [options]

Options:
  -p, [--port=port]                        # Runs Rails on the specified port - defaults to 3000.
  -b, [--binding=IP]                       # Binds Rails to the specified IP - defaults to 'localhost' in development and '0.0.0.0' in other environments'.
  -c, [--config=file]                      # Uses a custom rackup configuration.
                                           # Default: config.ru
  -d, [--daemon], [--no-daemon]            # Runs server as a Daemon.
  -e, [--environment=name]                 # Specifies the environment to run this server under (development/test/production).
  -u, [--using=name]                       # Specifies the Rack server used to run the application (thin/puma/webrick).
  -P, [--pid=PID]                          # Specifies the PID file.
                                           # Default: tmp/pids/server.pid
  -C, [--dev-caching], [--no-dev-caching]  # Specifies whether to perform caching in development.
      [--early-hints], [--no-early-hints]  # Enables HTTP/2 early hints.
```

As a bonus, if you mistype the server to use, you'll get an
auto-correction message:

```
$ rails s tin
Could not find handler "tin". Maybe you meant "thin" or "cgi"?
Run `rails server --help` for more options.
```

1 files changed, 18 insertions, 0 deletions

diff --git a/railties/CHANGELOG.md b/railties/CHANGELOG.md
index a38888afe5..335eb6c4e3 100644
--- a/railties/CHANGELOG.md
+++ b/railties/CHANGELOG.md
@@ -1,5 +1,23 @@
 ## Rails 6.0.0.alpha (Unreleased) ##
 
+*   Deprecate passing Rack server name as a regular argument to `rails server`.
+
+    Previously:
+
+        $ bin/rails server thin
+
+    There wasn't an explicit option for the Rack server to use, now we have the
+    `--using` option with the `-u` short switch.
+
+    Now:
+
+        $ bin/rails server -u thin
+
+    This change also improves the error message if a missing or mistyped rack
+    server is given.
+
+    *Genadi Samokovarov*
+
 *   Add "rails routes --expanded" option to output routes in expanded mode like
     "psql --expanded". Result looks like: