rp2040-chord-vco

This should functionally change nothing, but:

- makes code formatting, use of spaces and so on more consistent
- fixes some wrongly indented parts of the code
- renames some functions and variable to have them more cleary describe
  their purose
- adds some comments and move others around
- move all IO/Pins into #define variables that can be seen on top
- factor some parts of the code out into functions to make the loop more
  concise