Improve JS_DetectModule

[bnoordhuis]

It's still not infallible (I don't think it can ever be, the whole premise is wrong) but hopefully it's a little less fallible now.

Fixes: #606

[bnoordhuis]

examples/function_source.js and examples/hello.js are detected as modules now, which is not wrong but neither is it particularly right (can be parsed either way.)

I guess I could make js_parse_program and everything it calls track whether it saw syntax that is definitely ESM-only but that's:

1. a lot of effort, and

2. not really better, merely backwards compatible; it just flips the problem around of classic scripts getting classified as modules, now it's modules without definite ESM syntax getting misidentified (problematic when run in sloppy/non-strict mode)

[saghul]

I love me a beautiful hack!