Only add event listeners when a block is first mounted

[Rich-Harris]

This is an alternative to #4781, and fixes #4693. Rather than passing down a #remount variable that indicates whether a block is being remounted rather than mounted (due to a keyed each reordering), it allows blocks that contain event listeners (due to the on:, bind: or use: directives) to maintain their own #mounted state, and to add or ignore those listeners as appropriate.

(Initially I tried to reuse #current, but that's a little tricky — it becomes false when a block is outroing but still mounted, which means that an interrupted outro transition would result in listeners being reapplied, i.e. a memory leak, albeit less severe than the current situation.)

It does result in more code for those blocks. My gut says it's still a win compared to passing #remount around everywhere (certain in terms of the compiler source code), but I'm open to persuasion (or science) to the contrary.