build: mark date-holidays-parser as external

[KristjanESPERANTO]

date-holidays-parser is loaded at runtime and shouldn't be bundled. The external config tells rollup explicitly to leave it out.

No behavior change - the package was already loaded at runtime. This just removes the unresolved dependency warning during build.

Before

$ node --run build

> date-holidays@3.28.0 yaml
> node scripts/holidays2json.cjs


> date-holidays@3.28.0 build:only
> rollup -c


./src/index.js → lib...
(!) Unresolved dependencies
https://rollupjs.org/guide/en/#warning-treating-module-as-external-dependency
date-holidays-parser (imported by src/Holidays.js)
created lib in 132ms

After

$ node --run build

> date-holidays@3.28.0 yaml
> node scripts/holidays2json.cjs


> date-holidays@3.28.0 build:only
> rollup -c


./src/index.js → lib...
created lib in 131ms

[commenthol]

Hi @KristjanESPERANTO,
Good point to decouple this lib from the parser. IMHO this would be a breaking change as the bundled artifacts are commonly used from CDNs.
I'll postpone this for now to check possible side-effects first.

[KristjanESPERANTO]

Thanks for looking into this!

This PR only changes rollup.config.mjs, which builds lib/*.cjs (the require entry). That output already contains require('date-holidays-parser') today – rollup treats it as external implicitly and only prints a warning.

To verify, I built lib/ with the old and the new config and compared them. The output is byte-identical; the only difference is that the warning is gone.

The CDN/UMD bundles in dist/ come from webpack.config.cjs, which this PR doesn't touch, so they still bundle the parser as before.