feat(code-action): add quickfix to remove unused let bindings

[takeokunn]

Summary

Add code action to remove unused bindings from let expressions.

Example Transformation:

Before	After
let x = 1; in 2	let in 2
let x = 1; y = 2; in y	let y = 2; in y
let x = 1; y = 2; in 3	let y = 2; in 3 (each gets separate action)

Changes

File	Description
libnixf/src/Sema/VariableLookup.cpp	Generate fix for DK_UnusedDefLet diagnostics
nixd/lib/Controller/CodeAction.cpp	Set isPreferred: true for unused binding quickfix

Behavior

Action offered when:

• Cursor is on an unused let binding
• Diagnostic DK_UnusedDefLet exists

Action NOT offered when:

• Binding is used in the let body
• Binding is an inherit statement (intentionally excluded)

Handles removal:

• Removes full binding range including = value;
• Works with single-line and multiline let expressions

Test Plan

• All 4 unused-def tests pass
• Build succeeds with -Dwerror=true
• Tests cover:
  • basic.md: Remove single unused binding
  • multiple-bindings.md: Remove one of multiple bindings
  • all-unused.md: All bindings unused (each gets separate action)
  • multiline.md: Multiline let expression formatting
  • unquote-let-invalid.md: Interaction with unquote action (updated)
  • unquote-let-keyword.md: Interaction with unquote action (updated)

Related

• Issue: nixd: code actions #466
• Draft PR: [DRAFT] feat(code-action): add refactoring and quickfix actions for Nix code #755 (this implements Split 13)