[Browser FeatureReq] Allow extensions to pin action icons to the toolbar

This was discussed in the WECG. Extensions which rely on the action icon for their core functionality get common user feedback with questions asking how to open the extension on Microsoft Edge.

Unlike Safari and Firefox, Microsoft Edge does not allow extensions to specify the initial pinned state.

To accommodate those users and extension developers, this feature request suggests Microsoft Edge to follow Safari and Firefox in allowing an extension to set the default action icon state by specifying action.default_area="navbar" in the manifest.json file.

See action.default_area on mdn: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/action

cc @mukul-p