Add funcsAreBuiltins flag to show [native code] for JS builtins

[tmikov]

Summary:
ECMAScript requires that builtin functions return "[native code]" from
toString(). Many libraries (e.g., Babel) check for this string to detect
builtins and alter their behavior accordingly.

Previously, JS-implemented builtins from InternalJavaScript and extensions
would return "[bytecode]" (since their source is not available), causing
these library heuristics to fail.

Add a new funcsAreBuiltins flag to RuntimeModuleFlags that causes
Function.prototype.toString() to return "[native code]" for all functions
in the module. Set the flag when loading internal bytecode and extensions
bytecode.

Subtle logic for InternalJavaScript (Promise):
The funcsAreBuiltins flag for InternalJavaScript is set conditionally based
on hasMicrotaskQueue(). This is intentional:

Some React Native versions detect whether to use the VM's microtask queue by
checking Promise.toString().includes("[native code]"). When microtask queue
is disabled but Promise still reports as "[native code]", RN incorrectly
assumes the engine queue is enabled and skips its own queue implementation.

By making funcsAreBuiltins conditional:

• Microtask queue enabled: Promise shows "[native code]" (behaves like native)
• Microtask queue disabled: Promise shows "[bytecode]" (RN uses its own queue)

This ensures the RN heuristic produces correct results.

Note: The hermes CLI defaults to microtask queue enabled, so existing tests
continue to pass without modification.

Extensions always have funcsAreBuiltins=true since they don't affect the
microtask queue detection logic.

Differential Revision: D89523668

[meta-codesync]

@tmikov has exported this pull request. If you are a Meta employee, you can view the originating Diff in D89523668.

[meta-codesync]

This pull request has been merged in 79e312b.