fix webpack polyfills (#9735)
fix webpack polyfills (#9735)

webpack polyfills did not work because useBuiltIns: 'entry' expects a
explicit core-js import. Changed it to 'usage' which does not require
these explicit imports and polyfills based on browserslist.

As a result, the built index.js now went from 128kB to 192kB.

Ref: https://babeljs.io/docs/en/babel-preset-env#usebuiltins

Co-authored-by: zeripath <6a547fdd28db89120bdd70d4af1f9ba672ccd43a@cantab.net>

cez81/gitea,vCaesar/gitea,vdbt/gitea,vdbt/gitea,sapk-fork/gitea,lafriks/gitea,sapk-fork/gitea,sapk-fork/gitea,lafriks/gitea,cez81/gitea,cez81/gitea,lafriks/gitea,go-gitea/gitea,vCaesar/gitea,go-gitea/gitea,go-gitea/gitea,sapk-fork/gitea