xtask: rework example command structure

[MabezDev]

To run a single example currently this is the command: cargo xtask run example examples esp32s3 --example gpio_interrupt, which involves writing example three times.

I propose a new syntax:

cargo xtask run example esp32s3 gpio_interrupt

I'd also accept

cargo xtask run example gpio_interrupt --chip esp32s3

as a viable alternative (bonus points if we get a chip selector if we omit --chip

This new syntax assumes we're always running examples from the examples directory. This makes sense imo, in the 1.5 years we've had this xtask, I've never tried to run an example elsewhere.