Add an option to generate defmt support unconditionally

[honzasp]

This PR implements three different options for defmt support in generated code:

• DefmtOption::Disabled: code that implements defmt::Display is not included in the generated code (as already implemented).
• DefmtOption::Feature(feature): code for defmt::Display is gated behind a feature with given name (as already implemented).
• DefmtOption::Enabled: code for defmt::Display is included unconditionally (this option is new).

This is a breaking change in the API but backward compatible change in the CLI interface. I also added a CLI option --yes-defmt which corresponds to DefmtOption::Enabled (the opposite of --no-defmt, which maps to DefmtOption::Disabled).

This is useful when you want to include some code generated by chiptool into a larger crate where defmt is always available.