Refactor/Rethink JS Injector

[maxime-rainville]

Description

The JS injector is somewhat confusing and poorly understood.

We should aim to understand:

• what its purpose is and what's the actuar problem it's trying to solve
• how and where to use it
• how it's put together.

Following this, we may decide to remove it or replace it or just better document it

Additional context or points of discussion

No response

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)