update prepare.sh, xtp.toml & can remove pdk `main`

[nilslice]

If we update prepare.sh to require tinygo version 0.34+ (as well as whatever compatible go versions are supported), then we can remove the func main() {} stub we generate as it is no longer needed.

In xtp.toml, we need to then update the build command to include -buildmode c-shared param in the tinygo call.

[bhelx]

Did you pick this up already @nilslice ? If not, me or Muhammad can pick it up.

[nilslice]

@bhelx not yet -- but curious if your issue in #25 is related to the 0.34 version?

[nilslice]

(actually, nvm, I'm on 0.34 and can't repro)