Add support to detect functions that return `msg.sender` via direct or aliased return values

### Describe the desired feature

Introduce support in Slither to detect Solidity functions that return `msg.sender`, either directly or through transitive variable aliasing

## Motivation
Static analysis often requires understanding the flow of `msg.sender`—especially in:
- Authorization checks
- Meta-transaction handling (_msgSender() patterns)

Being able to detect whether a function returns `msg.sender`, even through variable aliasing, is a foundational capability for writing detectors related to access control, trust boundaries, and function exposure.