Improve code structure in Repo

[navneet1v]

With the merging of custom_import_maps and maps-layers plugin the code in the src and test folders are not abstracted in right form. Not only this the init code is also not abstracted properly. The ask of this issue is to enhance the code quality of this repo to have correct sub folders in the plugin for different maps enhancements.

Example: The file: custom_import_map/public/plugin.tsx can have 2 separate init functions for the 2 different features.

This will allow new users to add new functionalities and maintaining the code easy in future.