[Theme][Enhancement] Review ergonomics of theme selection flags in the push command

[jamesmengo]

          The entire treatment of the `-u` `-d`, `--live`, and `-t` flags are pretty smelly imo

• -u, -d, and --live represent the role of the theme, so I feel like the combination of these two flags can make usage confusing. Should combining theme push to ALL of those themes? - My initial thought is that it should create 3 themes OR we need to specify the precedence of these flags in the docs
• the -t flag changes meaning when you provide it in combination with -u and -d
  • theme push -t themename searches for a theme with the name themename and fails if it doesn't exist
  • theme push -u -t themename will create a theme with the name themename even if one already exists

This pr is not the place to apply these changes (in fact, it's kind of difficult to change the semantics of these flags without careful consideration), but I think the ergonomics of these flags can be fleshed out a little more :)

Originally posted by @jamesmengo in https://github.com/Shopify/cli/pull/3798#discussion_r1581616561