[💄Feature]: Drop bootstrap.

[wiseaidev]

👶 Getting Started
Please search the history to see if an issue already exists for the same problem.

• I have searched the existing issues.
• I have tested the latest version.

📝 Describe the feature
bootstrap was not heavily used throughout the code base. So, It is an optional dependency, and some classes can be replaced with good ol plain CSS.

📸 Screenshots
These are some examples that can be replaced with just plain CSS:

https://github.com/wiseaidev/flexify/blob/e7ca90034a543b030a9f7e58b9ebfccea80b5fc7/popup.html#L63

https://github.com/wiseaidev/flexify/blob/e7ca90034a543b030a9f7e58b9ebfccea80b5fc7/popup.html#L583

https://github.com/wiseaidev/flexify/blob/e7ca90034a543b030a9f7e58b9ebfccea80b5fc7/popup.html#L678

https://github.com/wiseaidev/flexify/blob/e7ca90034a543b030a9f7e58b9ebfccea80b5fc7/popup.html#L684

🔦 Context