Distance units for Parsec and Astronomical unit

[jschlyter]

I suggsst we add parsec (pc) and astronomical unit (au) as distance units. These are both non-US and non-metric units, and added for completeness and because they are interesting and fun. They are probably not applicable to most home automation installations unless you're running Home Assistant on the International Space Station or measuring distance to other galaxies.

Not adding these units will most likely have zero impact, but on the other hand adding them does not break anything (and will teach us to use very big and small numbers).

Closed PR in home-assistant/core#129187 if someone wants to take a look.

[AJediIAm]

I did a bit of searching and there are a few HACS integrations which could make use of this.

https://github.com/partofthething/ha_skyfield?tab=readme-ov-file
And
https://github.com/mawinkler/astroweather
Show the position of planets.
The Skyfield library https://rhodesmill.org/skyfield/ includes distance in au. By adding the units, the listed integrations could show the distance to the planets.