keep dynamic import when target set to es5

[hardfist]

Currently esbuild will transform dynamic import to require when target set to es5, which will cause problem when we want to further transpile the transpiled code to systemjs format(commonjs is not supported to systemjs).

[evanw]

I'm adding the ability to configure syntax feature support at the individual feature level in the next release. So you will be able to do --supported:dynamic-import=true to do this. I'm closing this issue because I believe this should now be fixed.