Previously, editor elements had to listen for mouse_up events to
determine when a click had completed. This meant that they only had
access to modifier keys that were pressed during the mouse_up event.

This led to some incorrect user experiences, such as executing a
ctrl+click if the user pressed ctrl after pressing the mouse button, but
before releasing it.

This change adds a click event handler to EditorElement, and adds a
modifier() method to the ClickEvent, which only includes the modifier
keys that were pressed during both mouse down and mouse up. The code for
handling link clicks has been moved into the click event handler, so
that it's only triggered when the non-multi-cursor modifier was held for
both the mouse down and mouse up events.

Closes #12752, #16074, #17892 (the latter two seem to be duplicates of
the former!)

Release Notes:

- Fixed a bug where pressing ctrl/cmd (or other modifiers) after mouse
down but before mouse up still triggered ctrl/cmd+click behavior (e.g.
"go to definition")