feat: add variadic defmacro parameters and macro helpers

[thsfranca]

Summary

• Add & rest variadic parameter support to defmacro — extra arguments are collected into an SList
• Add empty? and keyword? macro helpers to the AST evaluator
• Move prelude.vx from src/ to stdlib/ for language/compiler separation
• Update TopForm::DefMacro AST node with rest_param: Option<String> field

Test plan

• All 554 tests pass (8 new tests for variadic macros, helpers, and parser)
• cargo clippy clean
• Variadic macro collects excess args into rest param as SList
• Variadic macro with zero rest args produces empty list
• Variadic macro with fixed + rest params works correctly
• Error on too few args for variadic macro
• empty? and keyword? helpers work in macro bodies
• Parser handles [& items] and [a b & rest] syntax