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

[codecov]

Codecov Report

Merging #94 (2e742a8) into master (8d5c951) will increase coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   96.33%   96.71%   +0.38%     
==========================================
  Files           3        3              
  Lines         218      213       -5     
  Branches       87       81       -6     
==========================================
- Hits          210      206       -4     
+ Misses          8        7       -1     

Impacted Files	Coverage Δ
src/parse.ts	97.66% <100.00%> (+0.50%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[cmdcolin]

not a super big speedup but about a 20% faster (~13 vs ~16 seconds on my machine) compared to master branch on the vcf-bench https://github.com/brentp/vcf-bench

[cmdcolin]

one other improvement of this PR is a bit better typescript typing on the Variant object type, currently it's an "any"

[garrettjstevens]

For reference the suggested compiler options for Node 12 are:

{
  "compilerOptions": {
    "lib": ["ES2019"],
    "module": "commonjs",
    "target": "ES2019"
  }
}

From here: https://github.com/microsoft/TypeScript/wiki/Node-Target-Mapping

[cmdcolin]

updated :) i leave out module because i use it in the package.json --module field