Opt-in var declarations

Date: 2026-04-20 Area: runtime Pull Request: #368

Opt-in var declarations (--compat-var). Added var support behind a --compat-var CLI flag for ECMAScript compatibility when porting legacy code. Var bindings are stored in a separate FVarBindings map on function/module/global scopes (distinct from the lexical binding map used by let/const), matching the ES spec's separation between VariableEnvironment and LexicalEnvironment. This avoids forcing var semantics (function-scoped, redeclarable, no TDZ) through lexical machinery designed around block-scoping, TDZ, and no-redeclaration guarantees. The bytecode compiler uses a parallel DeclareVarLocal approach with depth-0 locals that survive EndScope unwinding. Naming follows spec terminology: DefineLexicalBinding / DefineVariableBinding / GetBinding / AssignBinding. language.md § var Declarations.