More Particle Xenon support

[jaswope]

Thanks for all the work you already put in on this! This PR addresses the following issues:

• Modified the variant definition so that all the examples will compile (basing the changes on choices made by other variants)
  • Defined the VBAT and AREF pins (A6 and A7)
  • Defined LED_RED and LED_BLUE, assigned them both to the onboard blue LED (same as done in other variants with only one status LED)
  • Defined LED_CONN
  • Changed LED_BUILTIN to a define to match other variants
• Added the variant to the github workflow
• DRY'd up the pin definitions to be more in line with other variants
• Used the correct Flash SPI device definition

[outlandnish]

Thanks, yeah these are definitely needed. I haven't really followed best engineering practices through this process so I really appreciate you taking the time to clean it up a bit more!