allow arbitrary expressions in each block keys

[Rich-Harris]

fixes #703

[codecov-io]

Codecov Report

Merging #1381 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1381      +/-   ##
==========================================
+ Coverage   91.17%   91.21%   +0.03%     
==========================================
  Files         120      120              
  Lines        4431     4428       -3     
  Branches     1363     1362       -1     
==========================================
- Hits         4040     4039       -1     
+ Misses        155      154       -1     
+ Partials      236      235       -1

Impacted Files	Coverage Δ
src/parse/state/mustache.ts	87.97% <100%> (+0.93%)	⬆️
src/compile/nodes/EachBlock.ts	98.02% <100%> (+0.01%)	⬆️

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