Expose set_breakpoint as a public API to the app being debugged

With this function exposed, the embedding script can do something like:
```lua
function debugbreak()
end

local info = debug.getinfo(debugbreak)
mobdebug.set_breakpoint(thisfile, info.lastlinedefined)
```
... and thus the debugbreak() function will cause the script to pause in the debugger.  This can be useful to put inside assert() failures, exceptions etc. to debug them in-situ.
Tested with zbstudio.