Add a component.outro method

[Rich-Harris]

I'm actually not certain that we need this, though I'm just going to leave it here anyway. I was thinking about #1211, but outroing nested components wouldn't be done with a public outro method (no sense in creating a ton of Promise objects unnecessarily, it would all happen behind the scenes with callbacks), so essentially this change would add extra bytes for the sake of being able to outro a component programmatically, which I'm not sure is a common need.

[codecov-io]

Codecov Report

Merging #1449 into master will decrease coverage by 0.1%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #1449      +/-   ##
==========================================
- Coverage   91.09%   90.99%   -0.11%     
==========================================
  Files         122      122              
  Lines        4492     4496       +4     
  Branches     1406     1411       +5     
==========================================
- Hits         4092     4091       -1     
  Misses        158      158              
- Partials      242      247       +5

Impacted Files	Coverage Δ
src/compile/nodes/Component.ts	92.82% <0%> (-1.67%)	⬇️
src/compile/dom/index.ts	95.31% <66.66%> (-0.69%)	⬇️

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