Add support v12 alpha

[nojaf]

• Allow selection of ReScript 12 alpha 5 and higher
• Skips selection of ReScript Core
• Updates rescript.json accordingly

[cknitt]

@nojaf Nice work! Thanks a lot!

One thing: 12.0.0-alpha.5 is now the default selection. Could you make it so that the default selection stays on the latest 11.1.x?

[nojaf]

@cknitt Yup, done!