feat(@angular/build): Support splitting browser and server stats jsonfiles for easier consumption - #33870

…files for easier consumption

This feature supports splitting out the browser and server stats json files so it's easier to inspect the bundle in various analyzers and addresses angular#28185 angular#28671. Today, everything gets dumped into a single file and it's nearly impossible to use without hours of `fix -> remove unused browser/server chunks -> analyze` and starting the loop all over again.

This feature implements the feature request I made in angular#28185, along with another developers request to see a stats json file for just the initial page bundle. I've tested this out in my own repository and it's already helped an incredible amount. This will be required to be in the next Major version as it will break any existing build pipeline that relies on a single stats.json file.

…ndler context

Track the target platform (`browser` or `server`) on individual bundler context results and introduce `BundleMergedContextResult` to maintain separate `browser` and `server` metafiles when merging.

This simplifies downstream consumers:
- Enables direct usage of `metafiles.browser` for browser-specific steps (budgets, i18n, chunk optimization, CommonJS checks, and post-bundle processing).
- Allows `extractLicenses` and `logBuildStats` to process all platform metafiles directly without merging them into a single structure.
- Removes redundant filtering of server bundles in budget calculations.
- Streamlines the `statsJson` file emission for browser and server targets.