These updates significantly improve IDE tooling for imports across the Discourse core codebase, and also for framework packages. The `@types/ember-*` packages are a temporary solution until we get onto Ember 5, which ships its types in the main package.
The previous approach of having jsconfig files in each package directory did work, but once you start adding all the possible interlinks between them, we hit the file count limit of VSCode's tooling (because it counts every file for every jsconfig its referenced in). Having one file at the root means that a single file can apply to all core packages and plugins.
Long-term, to get the same functionality for all themes/plugins, we may need to look at building/publishing a Discourse types package which can be added to theme/plugin package.json files for development purposes.
David Taylor