Update to Rollup 2 and Acorn 8

[andrewiggins]

Really love this tool! Wanted to update it to the latest version of rollup to take advantage of all the tree shaking improvements that have occurred since the v1 release line

[andrewiggins]

Results of test suite when running locally (since there isn't any CI that runts test automatically):

[hasan-li]

would be great to merge these changes, since rollup-plugin-virtual is deprecated and fails during npm ci cc.: @Rich-Harris

[Rich-Harris]

thank you! closed via #16