Deferred transitions

[Rich-Harris]

This implements another part of #1431. Transitions can now return a function, which says 'come back to me in a moment'. This allows coordination between transitions, without worrying about order, which in turn facilitates this sort of thing:

In this GIF, a pair of transitions (which happen to be called send and receive) essentially 'talk' to each other by sharing measurements — if a node with key 'c' outros and another with the same key intros, they can each generate a CSS animation that makes it look as though a single node is morphing, all while using custom transform behaviour for spinning and scaling.

There's an awkward gap left by the outroing node, if it's not the one at the bottom — that will be fixed with the final part of #1431.

[codecov-io]

Codecov Report

Merging #1453 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1453      +/-   ##
=========================================
+ Coverage   91.06%   91.1%   +0.04%     
=========================================
  Files         122     122              
  Lines        4523    4521       -2     
  Branches     1424    1422       -2     
=========================================
  Hits         4119    4119              
  Misses        159     159              
+ Partials      245     243       -2

Impacted Files	Coverage Δ
src/compile/dom/index.ts	96.06% <ø> (+0.71%)	⬆️
src/compile/nodes/Element.ts	93.33% <100%> (ø)	⬆️
src/compile/nodes/Component.ts	94.58% <100%> (+0.41%)	⬆️

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