support both bsconfig.json and rescript.json

[bobzhang]

related to #4621
Options:

• fuse bsconfig into json
• make it rescript.json

[cometkim]

Any plans to introduce more human-friendly alternative formats like YAML/TOML/JSON5?

[cometkim]

@cristianoc I'd love to work on this if there's still time until v11. Is v11 a good time to deprecate bsconfig.json and print a warning message?

[cristianoc]

Yes v11 could be a good time for this.