Enabling replace behavior and making insertion bit smarter

[hrescak]

First of all, I find myself using this plugin all the time and we're heavy Feather users at @prisma so kudos for making this! This PR is addressing some common steps I need to take almost every time I use the plugin.

• If I have an icon already selected, selecting an icon from the UI will replace it instead of placing a new one. This is useful when I accidentally select the wrong icon, or when I want to replace one that already exists.
• If I have a layer selected, it will try to place it in a place in the hierarchy that has a high chance of being correct (as opposed to the page, which almost never is):
  • If my selected layer accepts children (like a group or a frame), place the icon inside.
  • if my selected layer doesn't accept children (like a rectangle, or an ellipse), place the icon next to it in the hierarchy.

If this is desirable, the next PR I'd like to propose is matching the stroke color of a placed icon if replacing. I also addressed the console errors.