add jsoo tests

[haochenx]

add test to make sure jsoo compiled programs using prr would load successfully in Node.js.

concrete testing plan

• under folder nodejs_tests
• create a test case nodejs_loading_tests.ml using a executable stanza setting modes to js; and add a rule to run the resulting js file with nodejs while adding this rule to the dune target runtest
  • include simple usage of
    • Prr.Jstr, Prr.Jv
    • Prr.Fut
    • Prr.Tarray
    • Prr.Blob
    • Prr.Base64
    • Prr.Json
    • Prr.Uri
    • Prr.Console
  • ref on dune usage: https://github.com/kxcteam/kxclib-ocaml/blob/main/jsoo/unit_test/dune
• add / make sure that CI on GH runs the test added