feat: add extern fn declarations for JS FFI

Adds `extern fn` syntax for declaring external JavaScript functions:
  extern fn getElementById(id: String): Element
  extern fn getContext(el: Element, kind: String): CanvasCtx = "getContext"
  pub extern fn alert(msg: String): Unit

Changes across 11 files:
- Lexer: `extern` keyword
- AST: `ExternFnDecl` struct + `Declaration::ExternFn` variant
- Parser: parse `extern fn` with optional `= "jsName"` override
- Typechecker: register extern fn type signatures
- Interpreter: ExternFn value with clear error on call
- JS backend: emit extern fn calls using JS name (no _lux suffix)
- C backend: silently skips extern fns
- Formatter, linter, modules, symbol_table: handle new variant

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/linter.rs

@@ -403,6 +403,9 @@ impl Linter {
                 Declaration::Function(f) => {
                     self.defined_functions.insert(f.name.name.clone());
                 }
+                Declaration::ExternFn(e) => {
+                    self.defined_functions.insert(e.name.name.clone());
+                }
                 Declaration::Let(l) => {
                     self.define_var(&l.name.name);
                 }