SvgPcb layout template backend + netlister refactor

[ducky64]

• Add EXPERIMENTAL (alpha stage) layout template generator for svg-pcb, with example for SwitchMatrix.
  • Components with a defined template (extends SvgPcbTemplateBlock and defines _svgpcb_template) generates custom layout templates
  • Other components (not part of a component above or subtree) generate into footprints from the netlister
• Add refdes netlists to test suite
• Add barebones keyboard example
• Netlister refactor
  • Move refdes parsing into the Backend, and how it's handled into the netlist file generator (instead of embedded in the NetlistTransform)
  • Move netlist data structures (block, net, pin) into the NetlistGenerator (instead of the file generator), clean up the data structures to use more structured types (instead of strings) where possible
  • Pins treated differently from ports, netlister only considers IR-level connections
  • Clean up and improve net naming priority (in particular prioritizing link names), this simplifies some names
• Fix XIAO-ESP32C3 definition
• TransformUtil.Path improvements
  • .append_* supports multiple paths
  • .starts_with prefix check w/ unit tests
• Lots of cleanup of unit tests