Change retains annotation from using term arguments to using type arguments

[noti0na1]

Fix #22842

This PR updates the retains annotation to use type arguments instead of term arguments in order to streamline capture set representations and related type checking. Key changes include:

• Adjusting the looking up for capture variables in capture sets, by creating dummy term symbols.
• Removing term parameter for retains annotation and capsOf.
• Modifying annotation construction in RetainingType, CaptureAnnotation, and associated AST desugarings.

For example,

def f[C^](x: A^): B @retains[x.type | C] // B^{x, C}