Support freely configurable suffix for generated .js files

[cknitt]

Since ReScript 11.0.0-rc.6, the suffix for the generated js files is freely configurable (see rescript-lang/rescript#6472).

(Well, almost freely, see https://github.com/rescript-lang/rescript-compiler/blob/adb55d016252174237adb5689568ce88032ca2b9/jscomp/bsb/bsb_package_specs.ml#L58.)

[cknitt]

Given that create-rescript-app and the official installation instructions for v11 are now using .res.(m)js, it would be great to have this fixed.

[rolandpeelen]

Merged in #86