build: Integrated babel into build to make bundle compatible with ES5+

[slurmulon]

Changes

• Integrated @rollup/plugin-babel into the build so that the bundle contains polyfilled code that is compatible with ES5+.
• These changes prevent breaking syntax errors on older versions of Chrome, Firefox, Safari, etc.

Gotchas

• I noticed a small change to dist/vue-meta.d.ts, but I'm not sure why it was introduced. I can dig into this further if needed, it seems like it could impact TypeScript integrations.
  • Interestingly, I ran the build against v3.0.0-alpha.10 and noticed the exact same change, so this does not appear to relate to my change (perhaps it's due to an implicit version bump related to TypeScript).
• I assumed IE9 as a build target, and this seemed necessary (for some reason) to fully polyfill the code - also happy to experiment with this more if needed/preferred.
• The size of the bundle naturally increases due to polyfilling (273k -> 357k).

Issues

• Build contains ES5 incompatible code #773
• Vue-meta is not supported in Mobile Safari (iOS < 15) due to unsupported class method declaration with arrow function syntax #760 (related)