fix: copy filename_template ito split chunks

[henryqdineen]

Summary

This is a reimplementation of a PR I opened against webpack (webpack/webpack#19104). We found that the splitChunks.cacheGroups.{cacheGroup}.filename config option was not being respected for any split initial chunks created based on the maxSize-based deterministic grouping.

I added a stats test case which configures vendors cache group (with filename options) for anything in node_modules but with a small maxSize so it creates many little chunks. This is the diff for the test from before and after the fix in case it helps:

- Entrypoint main 5.88 KiB = 990.js 352 bytes 961.js 352 bytes 727.vendors.js 352 bytes main.js 4.85 KiB
+ Entrypoint main 5.88 KiB = 990.vendors.js 352 bytes 961.vendors.js 352 bytes 727.vendors.js 352 bytes main.js 4.85 KiB
  chunk (runtime: main) 727.vendors.js (id hint: vendors) 20 bytes [initial] [rendered] split chunk (cache group: vendors)
    ./node_modules/c.js 20 bytes [built] [code generated]
  chunk (runtime: main) main.js (main) 80 bytes (javascript) 2.86 KiB (runtime) [entry] [rendered]
    ./index.js 80 bytes [built] [code generated]
- chunk (runtime: main) 961.js (id hint: vendors) 20 bytes [initial] [rendered]
+ chunk (runtime: main) 961.vendors.js (id hint: vendors) 20 bytes [initial] [rendered]
    ./node_modules/b.js 20 bytes [built] [code generated]
- chunk (runtime: main) 990.js (id hint: vendors) 20 bytes [initial] [rendered]
+ chunk (runtime: main) 990.vendors.js (id hint: vendors) 20 bytes [initial] [rendered]
    ./node_modules/a.js 20 bytes [built] [code generated]

Alternatives considered

It seems like we could move this logic in to the split method on the Chunk struct but I'm not sure if that is preferred.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[JSerFeng]

Personally I think it's fine to move this to split method

[henryqdineen]

Thanks for the feeback @JSerFeng. I implemented your suggestion in 1ad61af