Record syntax within JS objects gets parsed as JS objects

[bobzhang]

Here's a quick repro case:

let foo = {"foo": {bar: 1}};

{bar: 1} should be parsed as a record but it gets parsed as a JS object instead.

https://reasonml.github.io/try?rrjsx=true&reason=DYUwLgBAZg9jEF4IG8BEsaoFwoEYEMAnHARgF8yBuIA

cc: @rickyvetter

moved from https://github.com/BuckleScript/bucklescript/issues/4253

[bobzhang]

I suggest we give a warning on this case