HTMLDocument.registerElement is deprecated.

Use `customElements.define` instead of `document.registerElement` see https://javascript.info/custom-elements
```
HTMLDocument.registerElement (file:///C:/Users/wilme/AppData/Local/atom-nightly/app-1.63.0-nightly1/resources/app.asar/static/index.js:160:12)
f (C:\Users\wilme\.atom\packages\script\node_modules\atom-space-pen-views-plus\lib\main.js:26:87233)
e.openTag (C:\Users\wilme\.atom\packages\script\node_modules\atom-space-pen-views-plus\lib\main.js:26:91114)
e.tag (C:\Users\wilme\.atom\packages\script\node_modules\atom-space-pen-views-plus\lib\main.js:26:90760)
Function.div (C:\Users\wilme\.atom\packages\script\node_modules\atom-space-pen-views-plus\lib\main.js:26:88521)
Function.content (C:\Users\wilme\.atom\packages\script\lib\header-view.js:48:19)
```

Start at lib/header-view.js:48, where the stack trace shows the affected content being created, then inspect atom-space-pen-views-plus/lib/main.js at the reported location. Verify how the deprecated HTMLDocument.registerElement call is reached and confirm that opening the header no longer produces the deprecation warning after the compatible registration path is used.

Written by the indexing model from the issue text.