Update Nim bindgen (May 2022)

[gustavolsson]

• Use latest bindgen from https://github.com/floooh/sokol/pull/665
• Put generated files in gen/ and extra nim files in extra/
• Generate bindings for sokol_glue.h
• Only have sokol implementation in one .c file (per module)
• Add Nim template to wrap sokol_main() and make sure NimMain() is run (see clear.nim example)
• Follow nim coding style
• Add tasks for running examples to sokol.nimble

[floooh]

Ok, merged.