fix(esbuild): minify exposes target config for es5 projects

[ycjcl868]

Close evanw/esbuild#1060

[codecov]

Codecov Report

Merging #571 (f4344df) into master (99c5eb0) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #571      +/-   ##
==========================================
+ Coverage   46.05%   46.09%   +0.03%     
==========================================
  Files          67       67              
  Lines        1598     1599       +1     
  Branches      435      436       +1     
==========================================
+ Hits          736      737       +1     
  Misses        861      861              
  Partials        1        1              

Impacted Files	Coverage Δ
packages/plugin-esbuild/src/index.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 99c5eb0...f4344df. Read the comment docs.

[sorrycc]

1. target 接收字符串或数组
2. https://umijs.org/plugins/plugin-esbuild 文档补充下

[ycjcl868]

1. target 接收字符串或数组
2. https://umijs.org/plugins/plugin-esbuild 文档补充下

ok