Introduce VUE_VERSION env variable for testing @vue/compat

What does this MR do and why?

This MR adds support for VUE_VERSION variable in webpack and jest

Following behavior is expected:

• default behavior (no VUE_VERSION) should be unchanged
• VUE_VERSION=2 should use Vue.js 2 + gitlab-ui from source. This is just for debugging purposes to detect issues with setting up our webpack/jest infrastructure and has no value besides that
• VUE_VERSION=3 uses Vue.js 3 + gitlab-ui from source (mostly for debuggin purposes)

How to set up and validate locally

• 🚀 yarn webpack should pass
• 🚀 VUE_VERSION=2 yarn webpack should pass
• ⚠ VUE_VERSION=3 yarn webpack should fail complaining only about Vue template compilations (this is intended because Vue.js 3 has differences in templates and will be addressed in next MRs)
• 🚀 yarn jest should pass
• 🚀 VUE_VERSION=2 yarn jest should pass
• ⚠ VUE_VERSION=3 yarn jest should run, failing here & there, but not about jest misconfiguration

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.