stabilize @std/dotenv

[kt3k]

part of #4600

• fully documented
• fully tested
• no ongoing breaking change discussion
• 1.0.0-rc.1
• wait 1 month
• 1.0.0 (stabilization)

[iuioiua]

After discussions with Yoshiya, we've decided to not stabilize @std/dotenv, as it has overlap with Deno's --env-file flag. However, @std/dotenv does provide the ability to load .env files at runtime for compiled scripts (compiled using deno compile). --env-file doesn't have this ability. So, while the stability of @std/dotenv could change in the future. Either way, closing as unplanned for now.