Group outro callbacks

[Rich-Harris]

Started having a go at #648, but ran aground — will take a larger rethink than I have time for right now. Just going to leave the failing test here

[Rich-Harris]

Okay, this is working. The code is a little inelegant, I may try and neaten it up in the morning.

This also fixes #1426.

[codecov-io]

Codecov Report

Merging #1157 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1157      +/-   ##
==========================================
+ Coverage   91.02%   91.15%   +0.13%     
==========================================
  Files         122      122              
  Lines        4488     4489       +1     
  Branches     1404     1405       +1     
==========================================
+ Hits         4085     4092       +7     
  Misses        158      158              
+ Partials      245      239       -6

Impacted Files	Coverage Δ
src/compile/nodes/EachBlock.ts	97.95% <ø> (ø)	⬆️
src/compile/nodes/IfBlock.ts	96.55% <ø> (ø)	⬆️
src/compile/nodes/Element.ts	92.87% <ø> (+0.24%)	⬆️
src/compile/dom/Block.ts	93.13% <0%> (+0.98%)	⬆️
src/compile/nodes/Component.ts	96.18% <0%> (+1.71%)	⬆️

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