upgrade acorn and parse as ES2018

[Conduitry]

There are a few different places where we're parsing with Acorn. I think it makes sense to parse as ES2018 in each place apart from src/generators/dom/index.ts which I think has to do with shared helpers. It might make sense to abstract this out somewhere so we don't have to remember to update as many places each year.

[Conduitry]

cc @jacwright From my brief tests on this branch, spread operators work fine both in the <script> tag and in the template.

[codecov-io]

Codecov Report

Merging #1182 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1182   +/-   ##
=======================================
  Coverage   91.75%   91.75%           
=======================================
  Files         127      127           
  Lines        4572     4572           
  Branches     1495     1495           
=======================================
  Hits         4195     4195           
  Misses        159      159           
  Partials      218      218

Impacted Files	Coverage Δ
src/parse/read/script.ts	100% <ø> (ø)	⬆️
src/parse/read/expression.ts	100% <ø> (ø)	⬆️
src/parse/read/directives.ts	73.43% <100%> (ø)	⬆️

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