build: upgrade ecmascript version

[uiolee]

What does it do?

Screenshots

Pull request tasks

• [ ] Add test cases for the changes.
• [x] Passed the CI test.

[github-actions[bot]]

How to test

git clone -b esversion https://github.com/hexojs/hexo.git
cd hexo
npm install
npm test

[coveralls]

Pull Request Test Coverage Report for Build 9662098311

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.478%

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Totals
Change from base Build 9168412934:	0.0%
Covered Lines:	9343
Relevant Lines:	9392

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💛 - Coveralls