chore: align number/float64/base/identity with namespace conventions

- Adds the missing `number` keyword (present in 21/28 (75%) of siblings
  in `number/float64/base`).
- Adds the missing `// MAIN //` section marker in `lib/main.js` (present
  in 26/27 (96%) of leaf siblings).

Author: claude

lib/node_modules/@stdlib/number/float64/base/identity/lib/main.js

@@ -18,6 +18,8 @@
 
 'use strict';
 
+// MAIN //
+
 /**
 * Evaluates the identity function for a double-precision floating-point number `x`.
 *

lib/node_modules/@stdlib/number/float64/base/identity/package.json

@@ -60,7 +60,8 @@
     "double",
     "double-precision",
     "dbl",
-    "float64"
+    "float64",
+    "number"
   ],
   "__stdlib__": {}
 }