Deprecate -bs-g bsc-flag

[ryyppy]

The bs-g flag has been an experiment for better data structure debugging before ReScript compiled to idiomatic JS data structures.

Since this feature actually caused a bug (#6495) with the newest v11 features, I think it's time to deprecate bs-g to not cause any troubles in the future.

When deprecated, the compiler may yield a warning. Providing the flag should not change any compiler behavior.

[TheSpyder]

There is a bit of discussion in #3716 of other use cases for -bs-g (it sets a DEBUG variable) and wanting an easier way to turn it on. Such as supporting multiple bsconfig.json files, which would also let us do nice things like have compiler warnings in development become errors in CI (I am forced to use sed to do this).

Personally I found -bs-g useful for a BuckleScript "conditional compilation" feature that is so old it only ever worked in ML syntax. I use this for my logging framework; in production it's all zero-cost %identity externals but when debug is on I get extensive console logging. I later extended this file with constants that increase the size of my fast-check property tests in production builds.

There are ways to solve my specific use of -bs-g with -bs-D, it's just a bit more awkward to configure.