fix(build): build vue-compat in default exports mode

[xanf]

f2fb8a5 introduced change in rollup configuration - output.exports for all packages was swapped from auto to named (to avoid warnings, I assume). Unfortunately this broke vue-compat commonJS package where single default export is expected to be spreaded so @vue/compat will be a drop-in replacement for vue.

Now, for example, when you require @vue/compat in node.js environment (for example when running vue-test-utils) you need to access defineComponent as require('@vue/compat').default.defineComponent instead of require('@vue/compat').defineComponent

Since @vue/compat is expected to serve as "drop-in" replacement for vue this PR is forcing output.expots = default for vue-compat package which is exactly the expected case - rollup expects single default export

[yyx990803]

Merged too fast - follow up in 22a9b37