repl: avoid deprecated require.extensions in tab completion

[gulbaki]

This PR replaces the usage of require.extensions in the REPL autocompletion logic with Module._extensions.

require.extensions has been runtime-deprecated under DEP0039, and its usage triggers a warning in recent Node.js versions. Since require.extensions is just a user-land alias for Module._extensions, we can safely access the same object via Module._extensions without any behavioral change or deprecation warning.

Changes:

• this.context.require.extensions → Module._extensions in the REPL complete() function
• Adds an inline comment explaining the deprecation and replacement
• No functional behavior change

Fixes: #58641