Runtime: shorthands for configuring "dev" and "prod" envs

[begelundmuller]

• This PR adds shorthands for configuring environment-specific overrides for the dev and prod environments.
• This saves one level of nesting for configuring our two primary environments.
• The shorthands work in both individual YAML files and for rill.yaml.
• We still support the current env:environment_name: syntax. This keeps environment-specific overrides available for custom environments.

Examples:

# sources/my-source.yaml
connector: s3
path: s3://bucket/**/*.parquet
dev:
  path: s3://bucket/year=2024/month=01/**/*.parquet

# rill.yaml
prod:
  models:
    materialize: true