Handle await block promises in a shared helper

[Rich-Harris]

Trying to tidy some of this stuff up in order to tackle #956. Will try and fix #1417 in this PR as well.

It results in slightly more code if you have one await block in an app, but less if you have two or more.

[codecov-io]

Codecov Report

Merging #1418 into master will increase coverage by 0.02%.
The diff coverage is 90.9%.

@@            Coverage Diff            @@
##           master   #1418      +/-   ##
=========================================
+ Coverage   91.18%   91.2%   +0.02%     
=========================================
  Files         122     122              
  Lines        4490    4481       -9     
  Branches     1386    1390       +4     
=========================================
- Hits         4094    4087       -7     
+ Misses        154     153       -1     
+ Partials      242     241       -1

Impacted Files	Coverage Δ
src/compile/dom/Block.ts	93.39% <ø> (ø)	⬆️
src/shared/utils.js	100% <ø> (ø)	⬆️
src/compile/nodes/AwaitBlock.ts	95.65% <90.9%> (+2.06%)	⬆️

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