HTMLDocument.registerElement is deprecated.

Use `customElements.define` instead of `document.registerElement` see https://javascript.info/custom-elements
```
HTMLDocument.registerElement (file:///C:/Users/Human/AppData/Local/atom/app-1.63.1/resources/app.asar/static/index.js:160:12)
f (C:\Users\Human\.atom\packages\script\node_modules\atom-space-pen-views-plus\lib\main.js:26:87233)
e.openTag (C:\Users\Human\.atom\packages\script\node_modules\atom-space-pen-views-plus\lib\main.js:26:91114)
e.tag (C:\Users\Human\.atom\packages\script\node_modules\atom-space-pen-views-plus\lib\main.js:26:90760)
Function.div (C:\Users\Human\.atom\packages\script\node_modules\atom-space-pen-views-plus\lib\main.js:26:88521)
Function.content (C:\Users\Human\.atom\packages\script\lib\header-view.js:48:19)
```

Start with lib/header-view.js at the content call shown in the stack trace, then inspect the openTag and tag paths in atom-space-pen-views-plus. Confirm where document.registerElement is reached and verify that using customElements.define removes the deprecation warning when the relevant view is opened.

Written by the indexing model from the issue text.