BUG: (type issue) cannot pass HTMLElement as icon when adding new rte action

GrapesJS version

• I confirm to use the latest version of GrapesJS

What browser are you using?

Chrome

Reproducible demo link

https://jsfiddle.net/vmeywbk8/11/

Describe the bug

Just a small type issue we came across - when adding an action to the RichTextEditor, the "icon" property of RichTextEditorAction is marked as string. But in addAction (https://github.com/GrapesJS/grapesjs/blob/793599cbd5493c0104db1134bd4b1684341274a7/src/rich_text_editor/model/RichTextEditor.ts#L363) if we pass in a HTMLElement, it will be appended to the button. This still works fine at runtime, but you will get a type mismatch error from TypeScript if you try to pass anything other than a string.

the "icon" property should probably be marked as "string | HTMLElement" to resolve this - I'd like to fix this as my first contribution

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