New semantic analyzer: refactor analysis of assignments #6412

This is a follow-up for #6390

Assignment statements can create a bunch of different symbol nodes:

• Variable
• Type alias
• Type variable
• Named tuple
• Typed dict

Because of this current implementation is messy and has some hacks. We should refactor this to happen in two steps:

• Classify the kind of assignment
• Perform analysis accordingly for each kind

The tricky corner case is enums: c = Color['Red'] looks like a type alias, but this should create a variable.