fix(build): format js/dune per dune fmt requirements

dune build @fmt was rejecting js/dune for two minor formatting issues:
single-line (preprocess ...) and (flags ...) stanzas. dune fmt prefers
them on multi-line. Reformatted; matches what 'dune build @fmt --auto'
would have produced.

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>

Author: hyperpolymath

js/dune

@@ -5,6 +5,8 @@
  (modes js)
  (modules playground js_api)
  (libraries affinescript js_of_ocaml)
- (preprocess (pps js_of_ocaml-ppx))
+ (preprocess
+  (pps js_of_ocaml-ppx))
  (js_of_ocaml
-  (flags (:standard --enable=effects --source-map))))
+  (flags
+   (:standard --enable=effects --source-map))))