feat: resolve let-in identifier patterns to their attrset bindings

[Alb-O]

Adds support for let ... in identifier patterns, where the body of a let-in expression is an identifier referencing an attrset binding

Motivation

Some Nix libraries define documented functions in let blocks and return them via an identifier:

let
  /** Doc comment */
  myFunc = x: x + 1;
  
  exports = { inherit myFunc; };
in
  exports

Previously, nixdoc couldn't extract documentation from these files because it only looked for attrsets directly in the let-in body, not identifier references to them.

Changes

• Added find_let_binding() to locate an AttrpathValue binding by name in a let block
• Added resolve_let_ident() to resolve identifiers with chained resolution support (e.g., alias = exports; in alias)
• Updated collect_entries() to check if the let-in body is an identifier and resolve it before collecting bindings
• Added tests for both direct and chained identifier resolution