refactor: convert defaults module to JSON in streams/node/join

Aligns `streams/node/join` with the sibling-package convention: 12/16
(75%) of `streams/node/*` packages with configurable options expose
defaults via a `lib/defaults.json` file and consume them with
`assign( {}, DEFAULTS )`. `join` was the lone outlier carrying an
equivalent `lib/defaults.js` function that returned a static object
literal — no dynamism, no semantic difference.

- Adds `lib/defaults.json` mirroring the literal returned by the
  removed `lib/defaults.js`.
- Removes `lib/defaults.js`.
- Updates `lib/main.js` to require the JSON file and use
  `@stdlib/object/assign` for a fresh copy, matching the pattern in
  `from-array`, `from-iterator`, `split`, and other siblings.

No change to observable behavior, public API, or test expectations.

Author: claude

lib/node_modules/@stdlib/streams/node/join/lib/defaults.js

@@ -0,0 +1,7 @@
+{
+  "objectMode": false,
+  "encoding": null,
+  "allowHalfOpen": false,
+  "readableObjectMode": false,
+  "sep": "\n"
+}

lib/node_modules/@stdlib/streams/node/join/lib/defaults.json

@@ -27,7 +27,8 @@ var Buffer = require( '@stdlib/buffer/ctor' );
 var setNonEnumerable = require( '@stdlib/utils/define-nonenumerable-property' );
 var setNonEnumerableReadOnly = require( '@stdlib/utils/define-nonenumerable-read-only-property' );
 var nextTick = require( '@stdlib/utils/next-tick' );
-var defaults = require( './defaults.js' );
+var assign = require( '@stdlib/object/assign' );
+var DEFAULTS = require( './defaults.json' );
 var validate = require( './validate.js' );
 var debug = require( './debug.js' );
 
@@ -163,7 +164,7 @@ function JoinStream( options ) {
 		}
 		return new JoinStream();
 	}
-	opts = defaults();
+	opts = assign( {}, DEFAULTS );
 	if ( arguments.length ) {
 		err = validate( opts, options );
 		if ( err ) {