Add an interactive REPL for Fantom (yarn fantom-cli)

[rubennorte]

Summary:
Adds yarn fantom-cli, an interactive REPL that evaluates JavaScript against the same native tester binary and Hermes runtime that Fantom tests run in. State persists across lines, input goes through Metro/Babel (so import, JSX and Flow all work), and the environment is set up the same way tests set it up — React, ReactNative and the Fantom API are available globally, so you can render surfaces and drive them interactively from the prompt.

The native tester gains an --interactive mode that loads a warm-up bundle without running tests and then evaluates length-prefixed snippets read from stdin, reporting results, console output and errors back as newline-delimited JSON. A Node driver hosts a Metro server, builds the warm-up bundle, spawns the binary, and bridges each line of input into the live runtime (top-level declarations persist across evaluations).

Features:

• Console-style output: results are printed with an inspector similar to the Chrome DevTools / Node.js consoles (nested objects/arrays up to a depth limit, quoted strings, functions/classes, Map/Set/RegExp/Date/Error, class instances, circular references, multi-line wrapping), colorized by type when stdout is a terminal. Inspecting a property never aborts the result or leaks into later evaluations: a property whose getter fails renders as [Thrown: <error>], including getters that fail asynchronously through the runtime's global error handler (e.g. accessing a react-native export backed by a TurboModule that isn't registered).
• Autocompletion: pressing Tab completes global identifiers, in-scope bindings and object properties (property names are listed without invoking getters).
• Node-like CLI: with no arguments it starts the REPL; -e <code> evaluates a snippet and exits; a filename runs that script and exits. In the non-interactive modes the value of a trailing expression is not printed (use console.log) and a thrown error exits with a non-zero status code.

Also documents the REPL in the Fantom README.

Changelog: [Internal]

Differential Revision: D110187712

[meta-codesync]

@rubennorte has exported this pull request. If you are a Meta employee, you can view the originating Diff in D110187712.