Support For Custom Extensions Module Loading

[kudla]

Hi. This allows to customize file extensions to be loaded for the Registry.

As an alternative it is really possible to handle this on a srcLoader level.
Still it could be useful to address this in general for the cases like

WithLoadExtensions(".mjs", ".js", ".json"),
WithLoader(func(name string) ([]byte, error) {
        if /* some custom condition */ {
            // some custom implementation
        }
        // Fallback. for default loader
        return require.DefaultSourceLoader(name)
}),

[dop251]

Hi. Thanks for the submission.

The current loading mechanism tries to follow the standard as close as possible. Any customisations or modifications should be left external.