Add pure-ESM build

[cmdcolin]

This is not super important but this PR adds a 'pure ESM' build
follows from my blog post https://cmdcolin.github.io/posts/2025-01-12-pureesm

• creates a pure esm build (we had a pseudo-esm "module" field in package.json before)
• adds potential ability to use the package without a bundler in the browser

[cmdcolin]

follows from being demonstrated here in vcf-js GMOD/vcf-js#108

has worked for awhile now