feat(code-action): add extract expression to file refactoring action

[takeokunn]

Summary

This PR adds a new "Extract to File" code action that extracts Nix expressions to separate files, automatically generating appropriate import statements with free variable handling.

Example transformation:

Before:

{ pkgs }: { buildInputs = [ pkgs.foo pkgs.bar ]; }

After extraction:

- New file (buildInputs.nix):
{ pkgs }:
[ pkgs.foo pkgs.bar ]
- Original file:
{ pkgs }: { buildInputs = import ./buildInputs.nix { inherit pkgs; }; }

Changes

File	Description
nixd/lib/Controller/CodeActions/ExtractToFile.cpp	Core implementation with free variable detection and unique filename generation
nixd/lib/Controller/CodeActions/ExtractToFile.h	Public interface declaration
nixd/lib/Controller/CodeAction.cpp	Integration with code action dispatcher
nixd/lib/meson.build	Build system integration
nixd/lspserver/include/lspserver/Protocol.h	LSP resource operations (CreateFile, RenameFile, DeleteFile)
nixd/lspserver/src/Protocol.cpp	JSON serialization for resource operations

Behavior

The action is offered when:

• Cursor is on a non-trivial expression (attribute sets, lists, lambdas, let bindings, if expressions, etc.)
• Cursor is on an attribute name (extracts the binding's value expression)

Free Variable Detection:

• Automatically detects variables used but defined outside the expression
• Wraps extracted code in a lambda with free variables as arguments
• Generates import ./file.nix { inherit var1 var2; } syntax
• Warns about with scope variables in action title (these may require manual adjustment)

Filename Generation:

• Uses binding key name if inside a binding (e.g., buildInputs.nix)
• Falls back to expression type (e.g., extracted-lambda.nix, extracted-attrs.nix)
• Automatically generates unique filenames (name-1.nix, name-2.nix) if file exists

Explicitly Excluded

• Trivial expressions: single variables, literals, paths, select expressions
• Empty attribute sets {}
• Empty lists []

Test Plan

• CI passes
• 4 regression tests covering:
  • Basic attribute set extraction (extract-attrs.md)
  • List expression extraction (extract-list.md)
  • Free variable detection (extract-with-free-vars.md)
  • with scope variable warning (extract-with-scope-vars.md)

Related

• Issue: nixd: code actions #466
• Draft PR: [DRAFT] feat(code-action): add refactoring and quickfix actions for Nix code #755 (this implements Split 6)