feat(code-action): add "convert to inherit" code action for bindings

[takeokunn]

Summary

Add a new code action that converts explicit bindings to inherit syntax.

Example transformations:

# Pattern 1: Simple variable assignment
# Before
{ foo = foo; bar = bar; }
# After
{ inherit foo; inherit bar; }

# Pattern 2: Select from source
# Before
{ x = lib.x; y = lib.y; }
# After
{ inherit (lib) x; inherit (lib) y; }

# Pattern 3: Nested source
# Before
{ x = lib.nested.x; }
# After
{ inherit (lib.nested) x; }

Features

• Only offered when cursor is on a binding node
• Supports simple { x = x; } pattern
• Supports { a = source.a; } pattern with arbitrary source depth
• Properly handles quoted attribute names (e.g., "foo-bar")
• Works in recursive attribute sets (inherit takes from lexical scope)

Edge Cases (action NOT offered)

• Multi-segment attribute paths: { foo.bar = baz; }
• Name mismatch: { foo = bar; }
• Selector mismatch: { foo = lib.bar; }
• Select with default value: { foo = lib.foo or null; }
• Dynamic/interpolated attribute names

Test Plan

• Added 9 regression tests covering:
  • Basic { x = x; } conversion
  • { a = lib.a; } conversion with source
  • Nested source { x = lib.nested.x; }
  • Quoted attribute names
  • Recursive attribute sets
  • Name mismatch (negative)
  • Selector mismatch (negative)
  • Multi-segment path (negative)
  • Default value (negative)

Related

• Issue: nixd: code actions #466
• Draft PR: [DRAFT] feat(code-action): add refactoring and quickfix actions for Nix code #755 (this implements Split 8)