Add apify env migrate #1576

One command that moves a project from environmentVariables and @secret references to .env.

Scope

• Move environmentVariables from actor.json into .env in the Actor directory.
• Resolve @secret references from ~/.apify/secrets.json. Stop on a missing secret and list the missing names. With --allow-missing-secrets, write those keys empty.
• Let keys that are already in .env win, and report the conflicts.
• Add .env to .gitignore, and remove the environmentVariables block from actor.json with jju, so formatting and comments survive.
• Print the next step: apify env push sets the values on the platform.
• Support --dir for monorepos and --yes.

Done when

• Tests cover plain values, @secret references, missing secrets, and an existing .env.

Depends on

• .env loading in apify run (Load .env in apify run #1569)