HTMLDocument.registerElement is deprecated.

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

Start at lib/header-view.js:48, where content triggers the stack trace, and inspect the atom-space-pen-views-plus call path shown in the report. Trace the document.registerElement usage and confirm the replacement is compatible with this Atom package. Done means opening the header view no longer produces the deprecation error.

Written by the indexing model from the issue text.