Fixes peggyjs#440.

CHANGELOG.md

@@ -53,6 +53,8 @@ Released: TBD
 - [#437](https://github.com/peggyjs/peggy/pull/437) Better type checking for visitor
 - [#438](https://github.com/peggyjs/peggy/pull/438) Make test build deterministic
 - [#439](https://github.com/peggyjs/peggy/pull/439) Make peg$computePosDetails a little faster
+- [#440](https://github.com/peggyjs/peggy/issues/440) Create directories for
+  output and source-map if they do not exist, rather than erroring.
 - [#446](https://github.com/peggyjs/peggy/pull/446) Add a right-associative `ExponentiationExpression` rule (operator `**`) to `javascript.pegjs` example grammar.
 - [#451](https://github.com/peggyjs/peggy/pull/451) Make stack.js ts clean
 - [#452](https://github.com/peggyjs/peggy/pull/452) Fixes to prepare generate-bytecode.js for ts-check