Use private fields for Variant class

[cmdcolin]

Private class fields are natively supported on nodejs v12+

Previously we were transpiling down to es5 to maintain nodejs compatibility, but es5 could not have private class fields transpiled down. But, in this PR, I propose --module commonjs instead of --target es5 for nodejs compatibility

There are a couple other small refactors but that is the main one

Possibly having less transpilation could improve some performance under node in e.g. vcf-bench. Not an urgent PR, just an idea