feat(core): support generator function as command action

[Anillc]

支持将 generator 传入 action 中

由于 microsoft/TypeScript#51187, 类型推断暂时并不能很好地工作

另外有待讨论的是 next 中传入一个 Promise<string[]>。相关 issue microsoft/TypeScript#44808

由于 arrow generator 还在 stage 1, 这个用法可能还不够优雅

[codecov]

Codecov Report

Base: 92.25% // Head: 92.39% // Increases project coverage by +0.14% 🎉

Coverage data is based on head (3a873d3) compared to base (e017de8).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #819      +/-   ##
==========================================
+ Coverage   92.25%   92.39%   +0.14%     
==========================================
  Files          60       60              
  Lines        6582     6605      +23     
  Branches     1276     1285       +9     
==========================================
+ Hits         6072     6103      +31     
+ Misses        510      502       -8     

Impacted Files	Coverage Δ
packages/core/src/command/command.ts	96.70% <100.00%> (+0.19%)	⬆️
packages/utils/src/misc.ts	72.91% <100.00%> (+2.46%)	⬆️
plugins/mock/src/client.ts	100.00% <0.00%> (+7.33%)	⬆️

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[purerosefallen]

How about RxJS observables?