Nested transitions

[Rich-Harris]

This is the next key part of the transition system overhaul after #1448. It adds a nestedTransitions compiler option, which, like skipIntroByDefault, is intended as a stopgap measure so that we can release this without having to bump the major version immediately.

TODO:

• replace 'countdown' method of scheduling outrocallback with something that requires less repetitive code (e.g. outrocallback = callAfter(outrocallback, 5) if there are 5 outroing elements in a block)
• handle EachBlock, AwaitBlock and Component, as well as IfBlock, plus IfBlock with else
• (probably separate to this PR) introduce some mechanism for preventing child transitions — I suggested this previously:

<!-- it would be weird if each <li> slid out while the <ul> itself was fading -->
{#if showList}
  <ul transition:fade childtransition:false>
    {#each list as item (item.id)}
      <li transition:slide>{item.name}</li>
    {/each}
  </ul>
{/each}

[codecov-io]

Codecov Report

Merging #1451 into master will increase coverage by 0.07%.
The diff coverage is 88.37%.

@@            Coverage Diff             @@
##           master    #1451      +/-   ##
==========================================
+ Coverage   90.98%   91.06%   +0.07%     
==========================================
  Files         122      122              
  Lines        4495     4523      +28     
  Branches     1410     1424      +14     
==========================================
+ Hits         4090     4119      +29     
- Misses        158      159       +1     
+ Partials      247      245       -2

Impacted Files	Coverage Δ
src/shared/utils.js	100% <ø> (ø)	⬆️
src/compile/nodes/EachBlock.ts	98.08% <100%> (+0.12%)	⬆️
src/compile/nodes/Element.ts	93.33% <100%> (+0.19%)	⬆️
src/compile/dom/index.ts	95.34% <100%> (+0.07%)	⬆️
src/compile/nodes/AwaitBlock.ts	96.47% <100%> (+0.12%)	⬆️
src/compile/nodes/IfBlock.ts	94.77% <57.14%> (-1.81%)	⬇️
src/compile/nodes/Component.ts	94.16% <85.71%> (+1.33%)	⬆️
src/compile/dom/Block.ts	93.45% <88.88%> (+0.38%)	⬆️

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