src: put bootstrappers in lib/internal/bootstrap/

Create `lib/internal/bootstrap/` and put bootstrappers there:

Before:

```
lib/internal
├── ...
├── bootstrap_loaders.js
└── bootstrap_node.js
```

After:

```
lib/internal
├── ...
└── bootstrap
    ├── loaders.js
    └── node.js
```

PR-URL: https://github.com/nodejs/node/pull/19177
Refs: https://github.com/nodejs/node/pull/19112
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/string_decoder.js

@@ -22,7 +22,7 @@
 'use strict';
 
 const { Buffer } = require('buffer');
-const { internalBinding } = require('internal/bootstrap_loaders');
+const { internalBinding } = require('internal/bootstrap/loaders');
 const {
   kIncompleteCharactersStart,
   kIncompleteCharactersEnd,