esp-generate improvements

[SergioGasquez]

I'll use this issue as a todo list/roadmap for esp-generate:

• Integrate Rhai for condition evaluation: Use Rhai as a library to evaluate if/else expressions.
• Improve conditionals:
  • Add else clauses
  • Allow nested conditions
• Deduplicate the code for different comment-styles (// vs #)
• Add rust-toolchain.toml file
• Add an argument to choose the template (blocking, async...) #27

[SergioGasquez]

Some of these issues are already resolved by #3, #4 and #5.