Support toggling display of a code host's native tooltips

Created by: lguychard

Fixes #4479

This avoids conflicts between GitHub's new jump to definition tooltips, and the Sourcegraph tooltips:

It introduces a new setting, codeHost.useNativeTooltips, and uses it to determine whether to show the code host's hover tooltips, or our own. By default, the browser extension will hide the code host's native tooltips, and display our own. If the user sets codeHost.useNativeTooltips to true in his settings, only the code host's hovers will be displayed.

The setting can be toggled from the command palette: