refactor: drop <8.x, support egg2. [BREAKING CHANGE]

[atian25]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

[codecov-io]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #5   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines          27     27           
=====================================
  Hits           27     27

Impacted Files	Coverage Δ
app.js	100% <100%> (ø)	⬆️

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[atian25]

@popomore 你发

[popomore]

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