Native spellcheck (nativeSpellcheck) doesn't work, has unnatural custom style, doesn't allow adding corrections to dictionary

Leaving the options empty or adding `spellChecker: false` and `nativeSpellchecker: true` should

1. make spelling mistakes appear as they would in [any ol' normal textarea](https://fishiecat.github.io/html-textarea-example/)
2. allow right clicking to add corrections to the browser spellchecker dictionary

EasyMDE doesn't. Unless [this 2020 comment](https://github.com/Ionaru/easy-markdown-editor/pull/143#issuecomment-599510773) is still valid.

**To Reproduce**

Below is the code based on and tested at https://stackblitz.com/edit/easymde :
```

EasyMDE demo

A spellink mistake.
Blueb erry wh ine.
Astount ing..

const easymde = new EasyMDE({

// START INSERTING

// END INSERTING
element: document.getElementById('my-text-area'),
sideBySideFullscreen: false
});

```

Paste the above into https://stackblitz.com/edit/easymde and add a space to the text area to trigger the spell check.

Then add
```
spellChecker: false,
nativeSpellchecker: true,
```
to the INSERTING commented area and trigger spellcheck again.

**Expected behavior**

1. I can add terms to the native spellchecker dictionary
2. The default, familiar browser style marking of spelling mistakes is used

Both are visible in [this ckeditor5 issue](https://github.com/ckeditor/ckeditor5/issues/9012)

Also here's a screenshot of both visible while writing this ticket:

> vvvvvvvvvvvvvvvvSCREENSHOTvvvvvvvvvvvvvvvv
> 
> ^^^^^^^^^^^^^SCREENSHOT^^^^^^^^^^^^^

**Actual behavior**

1. Adding terms to dictionary via context menu when right clicking on typos is not being offered
2. Nonstandard style to mark typos is used (pink background)

> vvvvvvvvvvvvvvvvSCREENSHOTvvvvvvvvvvvvvvvv
> ^^^^^^^^^^^^^SCREENSHOT^^^^^^^^^^^^^

**Version information**
- Current CDN version
- Windows Chrome and Firefox

Start with the provided StackBlitz reproduction and the EasyMDE configuration options `spellChecker` and `nativeSpellchecker`. Trace how those options affect the editor's browser spellcheck behavior, then verify that native typo styling and the dictionary context-menu action work in Chrome and Firefox.

Written by the indexing model from the issue text.