Improving module augmentation

[filipsobol]

We need to improve how we deal with module augmentation (explained in more details here), to:

• ensure that module augmentation kicks in when importing individual plugins from packages,
• all types are available in tests,
• avoiding arbitrary import './config' imports.

To encourage best practices and ensure that the .d.ts file with module augmentation is always imported, we will require importing all package contents from the main index file instead from individual files in the built package, eg.:

// ✔️
import { Table } from '@ckeditor/ckeditor5-table';

// ❌
import Table from '@ckeditor/ckeditor5-table/src/table';

[filipsobol]

Checklist for migrating packages to the new structure:

• Create src/augmentation.ts file.
• Search for all declare module '@ckeditor/ckeditor5-core' declarations and move them to the src/augmentation.ts as done in this file.
• Remove all import '../xxxconfig' imports.
• Add import './augmentation'; to the very bottom of the main index.ts.
• Replace imports in ckeditor5-build-* packages referencing individual files in the src of the package to use the package name instead:

// BEFORE
import Table from '@ckeditor/ckeditor5-table/src/table';

// AFTER
import { Table } from '@ckeditor/ckeditor5-table';

[filipsobol]

Below is a summary of the changes introduced in PR #13568 to fix module augmentation and testing within monorepo. The same changes (according to the checklist above) will be applied to all packages using module augmentation.

Module augmentation not being loaded in all cases

Previously, module augmentation were spread across many files and not visible in some other files. To mitigate this, we added import 'xxxconfig'; declarations to load files with needed augmentations. However, because the imports in the source .ts files and the output .d.ts files are different, the code that worked fine during the development didn't always work when the package was built and released. 

To mitigate this, we created an augmentation.ts file with augmentations that cover all the needs during the development. Since it's also imported in the main index.ts file, it will always be loaded as long as code from this package is not loaded from the src folder, e.g:

// BEFORE
import Table from '@ckeditor/ckeditor5-table/src/table';

// AFTER
import { Table } from '@ckeditor/ckeditor5-table';

Module augmentation in tests

We decided NOT to update import paths in tests, because tests often import internals that we don't want to export in the main index.ts. However, this caused TypeScript errors.

To mitigate this, we added "./packages/*/src/augmentation.ts" and "./external/*/packages/*/src/augmentation.ts" to tsconfig.test.json to force TypeScript to always load all available augmentation files when running the tests.
 

Fix loading of tsconfig.json in manual and automated tests

While investigating issues with the tests, we noticed that it loads seemingly arbitrary tsconfig.json files. Once this issue was fixed, we no longer needed to have tsconfig.json (used for testing and loaded by the IDE) and tsconfig.release.json (used for building) for each package. Since tests are always run from the root-level of the whole project, we now have tsconfig.test.json file used specifically for this purpose, and each package now has only have a single tsconfig.json file used by the IDE and for building.

[Reinmar]

Status page of porting all the packages to the new format: https://chill-saturn-f1f.notion.site/Augmentation-migration-62971d5b535e4f7c982fe78d2acd3f0e