Improve and fix asset emission

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

This will resolve some important issues when emitting assets and make it more configurable at the same time. The most important fix is that for some formats, generated asset URLs would point to a file in the parent directory instead of the actual asset. Also, the generated code for asset URLs has been refined, similar to the refinements for import.meta.url in #2785:

• all formats except cjs and umd assume the code runs in a browser environment where URL and document are available
• for cjs and umd, presence of a browser environment is detected by checking for document (after which presence of URL is assumed)

Generated code:

// amd
new URL(module.uri + '/../relative/path/to/asset.svg', document.baseURI).href

// cjs
(typeof document === 'undefined' ? new (require('u' + 'rl').URL)('file:' + __dirname + '/relative/path/to/asset.svg').href : new URL((document.currentScript && document.currentScript.src || document.baseURI) + '/../relative/path/to/asset.svg').href)

// es
new URL('relative/path/to/asset.svg', import.meta.url).href

// iife
new URL((document.currentScript && document.currentScript.src || document.baseURI) + '/../relative/path/to/asset.svg').href

// system
new URL('relative/path/to/asset.svg', module.meta.url).href

// umd
(typeof document === 'undefined' ? new (require('u' + 'rl').URL)('file:' + __dirname + '/relative/path/to/asset.svg').href : new URL((document.currentScript && document.currentScript.src || document.baseURI) + '/../relative/path/to/asset.svg').href)

To be able to control and optimize this code, there is now a new plugin hook

resolveAssetUrl: ({assetFileName: string, relativeAssetPath: string, chunkId: string, moduleId: string, format: string}) => string | null

[guybedford]

I can't say I've done a thorough review, but the hook seems useful, and that it fixes what sounds like a critical bug seems pretty important. Great work.