[Wrangler] Please document the --var and --define arguments

[rozenmd]

Discussed in cloudflare/workers-sdk#1807

^{Originally posted by AaronBeaudoin September 9, 2022}
By running wrangler dev -h I found this:

--var        A key-value pair to be injected into the script as a variable  [array]
--define     A key-value pair to be substituted in the script  [array]

Some more explanation regarding these would be appreciated. A few points:

1. Neither of these arguments are anywhere at https://developers.cloudflare.com/workers/wrangler/commands.
2. When I tried using --var, it took me some time to figure out why TEST=testing didn't work. Turns out the =, which seemed like the natural character, actually needed to be a :. It seems this is currently only discoverable by trial and error.
3. It's not clear to me what --define does. Some docs with example code would go a long way towards clearing these up.

Thanks so much!