feat: chunk importmap

[bhbs]

It seems that many people have been wanting improvements in chunk cache efficiency, so I've created this PR. If you could, please take a look and review this! Thank you Vite team!!

Description

Fixes: #6773 
Related: #15372
Related: #15373

Creating an importmap for chunks helps prevent cascading cache invalidation. This importmap features a list of filenames with file ID-based hashes linked to filenames with content-based hashes. When one chunk references another, it uses the ID-hashed filename instead of the content-hashed filename. As a result, only the updated chunk requires cache invalidation in the browser, leaving intermediary chunks unchanged. This strategy enhances the cache hit rate following deployments.

Implementation

During the renderChunk step, file hash placeholders in the code are overwritten with stable ID hashes. This process maintains a record of the hash placeholders and their corresponding ID hashes. (After that, during the augmentHash step, filenames are assigned based on the content.)

Based on the generated chunk list and the record of hash placeholders and their corresponding hashes, an importmap is generated and inserted into the HTML.

Others

• An importmap is generated for legacy bundles using the same method, but a systemjs-importmap is inserted instead of a standard importmap.
• Consider the importmap during the CSS processing (cssPostPlugin).
• Consider the importmap during import analysis (buildImportAnalysisPlugin).
• Only one importmap can be loaded at a time (MDN). If an importmap already exists, it needs to be merged (buildHtmlPlugin).

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