[TS] Fix importing module augmentations

[arkflpc]

We use module augmentation for adding properties to interfaces from ckeditor5-core with information provided by other packages:

• EditorConfig - plugin-specific config is added there,
• PluginsMap - it enables editor.plugins.get() to provide specific return type based on plugin name,
• CommandsMap - it enables editor.commands.get() to provide specific return type based on command name and also checks the parameters of editor.execute().

Return types are correctly provided at each call-site ({config,plugins,commands}.get()) only when the file with the proper declare module '@ckeditor/ckeditor5-core' is imported.
However, it's not always imported directly, and the results are not-consistent. Visual Studio Code usually sees those augmentations and reports no error. But, when running automatic tests, the compilation fails randomly with TypeScript emitted no output error.

Let's make sure that every augmentation is directly and explicitly imported:

1. Whenever there is plugin.get(), the type of the plugin is imported. Make sure that PluginsMap is augmented in the same file as plugin class.
2. Whenever there is commands.get(), the type of the command is imported. Make sure that CommandsMap is augmented in the same file as command class.
3. Whenever there is config.get(), the plugin config should be imported. Ideally, the config should be in a single file like ckeditor5-mypackage/src/mypackageconfig.ts.