feat: add option to customize asset file names

[ShGKme]

Resolves

mergeConfigs doesn't work for build.rollupOptions.output[].assetFileNames():

• In the lib config it merges output arrays and results in both default and new assetFileNames, duplicating assets
• In the app config it just replaces appConfig.ts values, requiring to override all cases, not only one specific, or duplicate this config defaults.

This PR adds new option assetFileNames to manually merge custom assetFileNames with the default. If custom one returns undefiend (falsy), then the default one is used.

Example

If a lib has styles and its entrypoint is main.ts, then styles are built to main.css and it is not possible to change.

---

@susnux If this options is fine, what do you think about adding the same for chunk file names to not more everything to one folder?

[codecov]

Codecov Report

Merging #41 (b1e4978) into main (63c6365) will increase coverage by 2.11%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
+ Coverage   79.70%   81.81%   +2.11%     
==========================================
  Files           8        8              
  Lines         547      572      +25     
  Branches       32       46      +14     
==========================================
+ Hits          436      468      +32     
+ Misses         79       58      -21     
- Partials       32       46      +14     

Files	Coverage Δ
lib/baseConfig.ts	87.09% <100.00%> (+0.79%)	⬆️
lib/appConfig.ts	80.00% <62.50%> (+1.05%)	⬆️
lib/libConfig.ts	90.72% <62.50%> (+1.91%)	⬆️

... and 1 file with indirect coverage changes

[susnux]

I think this is not a good example as you will have to set cssCodeSplit to false. Otherwise the styles would override each other.

[ShGKme]

The example was based on nextcloud/password-confirmation, where currently styles are renamed manually by adding && mv ./dist/main.css ./dist/style.css (only in a production build, dev build just doesn't work).

There is no cssCodeSplit and this custom function works.

But there is only one entrypoint