Markdown widget does not support content modification by Grammarly

**Describe the bug**

**Note:** This _could_ be an issue with Grammarly and _not_ NetlifyCMS, but this is the only platform that I've experienced this issue with. It does not occur with any other markdown/ WYSIWYG/rich text editor I've used.

Text in the `markdown` widget is not updateable by Grammarly. Selecting the extension's suggested grammar fixes dismisses the error notification that the extension provides while not updating the content.

However, text input in other widgets works just fine with Grammarly.

Since a lot of writers and publishing editors often use Grammarly as part of their editorial workflow, this could be an annoyance or even accidentally introduce errors into content when the uncareful eye sees the error underline disappear after accepting the fixes proposed by Grammarly.

**To Reproduce**
1. Have [Grammarly](https://www.grammarly.com/p) installed
2. Intentionally misspell something in a `markdown` block
3. Choose the suggested fix
4. Verify that the content doesn't update with the suggested fix

**Expected behavior**
`markdown` widget content should be updatable by browser extensions.

**Applicable Versions:**

- Netlify CMS version: [2.10.76]
- Git provider: [GitHub]
- OS: [macOS Catalina 10.15.7]
- Browser version [Brave 1.17.73]
- Node.JS version: [15]

**CMS configuration**
[real world example of an affected config.yml](https://github.com/tonyketcham/puerh.wtf/blob/f60b29395c72f54e1cadbf7c0c2c6347df3bf205/src/cms/config.yml#L412)