feat: Update GenerateConfig to make it immutable

[iamkun]

这个PR主要是修改了调用 moment API 时的写法，返回计算结果而不是其自身，更加贴近 immutable 的概念，也能让 https://www.npmjs.com/package/antd-dayjs-webpack-plugin 摆脱 badMutable 插件，真正实现 immutable

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[codecov]

Codecov Report

Merging #42 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   99.54%   99.54%   -0.01%     
==========================================
  Files          43       43              
  Lines        1773     1764       -9     
  Branches      518      519       +1     
==========================================
- Hits         1765     1756       -9     
  Misses          6        6              
  Partials        2        2

Impacted Files	Coverage Δ
src/generate/moment.ts	100% <100%> (ø)	⬆️

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