feat: implement LSP server for IDE integration

Add a Language Server Protocol (LSP) server to enable IDE integration.
The server provides:

- Real-time diagnostics (parse errors and type errors)
- Basic hover information
- Keyword completions (fn, let, if, match, type, effect, etc.)
- Go-to-definition stub (ready for implementation)

Usage: lux --lsp

The LSP server can be integrated with any editor that supports LSP,
including VS Code, Neovim, Emacs, and others.

Dependencies added:
- lsp-server 0.7
- lsp-types 0.94
- serde with derive feature
- serde_json

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/main.rs

@@ -5,6 +5,7 @@ mod diagnostics;
 mod exhaustiveness;
 mod interpreter;
 mod lexer;
+mod lsp;
 mod modules;
 mod parser;
 mod schema;
@@ -67,8 +68,31 @@ fn main() {
     let args: Vec<String> = std::env::args().collect();
 
     if args.len() > 1 {
-        // Run a file
-        run_file(&args[1]);
+        match args[1].as_str() {
+            "--lsp" => {
+                // Run LSP server
+                if let Err(e) = lsp::LspServer::run() {
+                    eprintln!("LSP server error: {}", e);
+                    std::process::exit(1);
+                }
+            }
+            "--help" | "-h" => {
+                println!("Lux {} - A functional language with first-class effects", VERSION);
+                println!();
+                println!("Usage:");
+                println!("  lux              Start the REPL");
+                println!("  lux <file.lux>   Run a file");
+                println!("  lux --lsp        Start LSP server (for IDE integration)");
+                println!("  lux --help       Show this help");
+            }
+            "--version" | "-v" => {
+                println!("Lux {}", VERSION);
+            }
+            path => {
+                // Run a file
+                run_file(path);
+            }
+        }
     } else {
         // Start REPL
         run_repl();