Move sequentialcreditnotes under `ext/` folder

Maybe this is a bit of bikeshedding, but..

* `civicrm/ext/` is used for similar content in a few other cases (eg when adding `iatspayment` to the tarball; eg for
   the `drupal-case` and `wp-case` build-types).
* `civicrm/ext/` looks analgous to `sites/default/files/civicrm/ext`, which has similar content.
* Among the list of folders under `civicrm/`, `ext/` looks more consistent.  There are only two
  folders with capital letters (`CRM/` `Civi/`) and only two folders with multiple
  words (`bower_components/` `release-notes/`)... and 18 other folders which are
  lowercase+single-word (`ang/` `api/` `bin/` `css/` `distmaker/`
  `extern/` `i/` `install/` `js/` `l10n/` `packages/` `settings/` `sql/`
  `templates/` `tests/` `tools/` `vendor/` `xml/`).
* The file-hierarchy is additive (combining elements of the OS/host FHS, of the CMS, and of intra-extension content)
  and makes for a long total.