New build target in v2

[koba04]

SWR v2 is a great opportunity to rethink our build target setting because changing the build target is definitely a breaking change. Currently, the build target is ES5, which is the target version of bunchee.

But IE is now officially out of support, which means we no longer have to support the ES5 environment.

I've tested how the build setting affects the bundle size of swr.

target=ES5
26490  7 31 22:48 index.js

target=ES2015
22832  7 31 22:54 index.js

target=ES2017
21302  7 31 22:52 index.js

target=ES2020
21302  7 31 22:53 index.js

I think ES2017 is the best target version for us.

To change the version, we have to change the target setting that bunchee has.

huozhi/bunchee#45

[koba04]

The build target of SWR v2 is ES2018 🚀 🚀
#2249