[core] fix(IconProps): widen type of HTML attributes

[adidahiya]

Changes proposed in this pull request:

Fix a regression introduced by #6235 where consumers' usage of HTML attribute props on <Icon> stopped compiling successfully due to the more specific React.HTMLAttributes<T> type. This caused compilation errors such as:

  Types of parameters 'evt' and 'event' are incompatible.
    Type 'MouseEvent<HTMLElement | SVGSVGElement, MouseEvent>' is not assignable to type 'SyntheticEvent<HTMLSpanElement, Event>'.
      Types of property 'currentTarget' are incompatible.
        Type 'EventTarget & (HTMLElement | SVGSVGElement)' is not assignable to type 'EventTarget & HTMLSpanElement'.
          Type 'EventTarget & SVGSVGElement' is not assignable to type 'EventTarget & HTMLSpanElement'.
            Type 'EventTarget & SVGSVGElement' is missing the following properties from type 'HTMLSpanElement': accessKey, accessKeyLabel, autocapitalize, dir, and 20 more.

250                     <Icon icon={IconNames.MORE} onClick={this.handleIconClick} />

The fix in this case is to widen the type of the element param in React.HTMLAttributes<T> which was inadvertently narrowed in #6235. Turns out that HTMLElement is really just an alias for Element (it adds no extra properties), so we were working with a pretty basic type in the first place.

Reviewers should focus on:

Fixes the regression. I verified this in consumer code by editing Blueprint's .d.ts files in node_modules.