feat: Add allow_js attribute

[jfirebaugh]

This allows swc to more faithfully mimic the behavior of tsc when used as a transpiler with ts_project. A ts_project wrapper macro can do:

    transpiler = partial.make(
        swc,
        out_dir = out_dir,
        root_dir = root_dir,
        allow_js = allow_js,
        source_maps = source_map,
        ...,
    )