This diff:
* introduces the `VersionedContentMerger` trait, which allows for
merging the updates of versioned contents within the same chunk group;
* implements this for `EcmascriptChunkContent`/`EcmascriptChunkUpdate`,
turning them into
`EcmascriptMergedChunkContent`/`EcmascriptMergedChunkUpdate`;
* creates a new `ChunkList` asset which is capable of merging chunk
updates of chunks within the same chunk group, and create such an asset
for dynamic chunks (through manifest/loader_item.rs) and chunk group
files assets.
This fixes a bunch of edge cases related to HMR:
* HMR of dynamic imports now works;
* Chunks getting added/deleted/renamed now also works with HMR, since
we're listening to updates at the chunk group level;
* CSS chunks get reloaded in the right order, with respect for
precedence.
There are still known edge cases with HMR:
* CSS chunks added through HMR are not inserted at the right position to
respect precedence (WEB-652).
* Update aggregation is disabled because we don't want a critical issue
to stop all HMR (WEB-582) . This would be fixed by applying aggregated
updates when dismissing the error modal, but there are some edge cases
with this too (e.g. what happens when an HMR update also causes an error
on top of an existing error).
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