Detect async arrow thenable side effects

[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:
Resolves #4116

Description

This will also detect side effects from accessing the then property or calling it when executing an async arrow function.

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#gh-4116-async-arrow-thenables

or load it into the REPL:
https://rollupjs.org/repl/?pr=4120

[codecov]

Codecov Report

Merging #4120 (09bb382) into master (5c05997) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4120   +/-   ##
=======================================
  Coverage   98.11%   98.12%           
=======================================
  Files         201      201           
  Lines        7068     7077    +9     
  Branches     2067     2071    +4     
=======================================
+ Hits         6935     6944    +9     
  Misses         64       64           
  Partials       69       69           

Impacted Files	Coverage Δ
src/ast/nodes/ArrowFunctionExpression.ts	100.00% <100.00%> (ø)
src/ast/nodes/shared/FunctionNode.ts	100.00% <100.00%> (ø)

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[kzc]

lgtm