Implement env.vars library with dotenv support

[deviaze]

• Implements @std/env/vars library
• Deprecates existing env.getvar/etc functions with new deprecate function. Runtime warnings can be disabled by env variable.
• seal now loads environment variables from a .env file if present with the dotenvy crate. This behavior can be disabled by turning SEAL_LOAD_DOTENV=false.
• Environment variables can be added at runtime by calling env.vars.load with a file and override param.
• fixes silly :3 mistake in jit pr that meant jit couldn't be disabled

Closes #42
Closes #92