Update to acorn 7

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

This will update to the next major acorn version. Breaking change on acorn side is that Import has been replaced by ImportExpression with different semantics, which requires the dedicated PR.

[codecov]

Codecov Report

Merging #3061 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3061      +/-   ##
==========================================
+ Coverage   88.73%   88.75%   +0.02%     
==========================================
  Files         165      165              
  Lines        5734     5737       +3     
  Branches     1748     1748              
==========================================
+ Hits         5088     5092       +4     
  Misses        388      388              
+ Partials      258      257       -1

Impacted Files	Coverage Δ
src/ast/nodes/index.ts	100% <ø> (ø)	⬆️
src/ast/nodes/NodeType.ts	100% <ø> (ø)	⬆️
src/ast/nodes/MetaProperty.ts	100% <100%> (+2.04%)	⬆️
src/ast/nodes/ImportExpression.ts	100% <100%> (ø)
src/Module.ts	94.19% <100%> (ø)	⬆️

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