[Vite] Template contents for Lit-/PolymerBundleParser (StatsPlugin)

[gilberto-torrezan]

We need to be able to read the template sources (which are in the frontend folder)
both in development and production mode

How it works today

Webpack creates a stats.json file containing the actual sources for all files included in the compilation
In dev mode, stats.json are read through the webpack dev server
In production mode, stats.json is read from the classpath
We drop unnecessary stuff from stats.json to make it smaller

How it could work with Vite

In dev mode, we can read the template directly from the Vite dev server (we could do this for webpack also if we want)
In production, copy all template sources to a resources directory (META-INF/VAADIN/frontend) and read them from the classpath on demand

Webpack generated a stats.json containing all template sources that can be requested so that we can parse the template contents. (See stats-plugin)
Template sources are requested through: FrontendUtils::getStatsContent
from which the sources in received stats.json content is searched with BundleLitParser::getSourceFromStatistics or BundleParser::getSourceFromStatistics for PolymerTemplate

Source requests are made in NpmTemplateParser and LitTemplateParserImpl as a possible place to get template sources in the method getTemplateContent.

Suggested changes

• Alias Frontend -> frontendFolder in vite.config.ts
• Template source can be retrieved directly from Vite by a direct request of filename (instead of by stats.json parsing with Webpack). The DevModeHandler interface currently provides a method (prepareConnection) which directly opens a connection the Webpack dev-server (used by e.g. FrontendUtils::getStatsContent) meaning that Webpack specifics are not abstracted by the interface. Need to add a method to DevModeHandler interface for retrieving template source so that the Webpack / Vite - specific details can be hidden inside the WebpackHandler / ViteHandler interfaces respectively.
• In production mode, the current implementation reads stats.json from either the classpath (default) or from an external URL (primarily added to support portlets). With Vite, we need to also store the template sources (in some format) so that they are available at runtime.
• During build-frontend with Vite, copy all template sources to a resources directory.
• In BundleLitParser, decouple the parsing of stats.json from the parsing of the template itself so that the first can vary (Webpack / Vite). This could be done by either having an interface with two implementations, something like TemplateSourceAccess / WebpackTemplateSourceAccess / ViteTemplateSourceAccess (or just an if-condition on the feature flag). Effectively, BundleLitParser::getSourceFromStatistics would go to WebpackTemplateSourceAccess and the Vite implementation would just simply read the template sources outright from the classpath.

[joheriks]

Fixed by #12204