Split the Environment class into a contract and traits #614

Summary

Split the Environment class into a contract and traits, as other packages do, with no behavior change. The typed readers that go into the new contract are tracked separately in #615.

Why

Quantum\Environment\Environment is one 213-line class with five jobs: state and loading, the app environment name, reading values, writing .env, and locating and parsing the files. It has no interface, unlike Config and Mailer, and the readers planned in #615 need a contract to be defined in.

Proposed change

src/Environment/
├── Contracts/EnvironmentInterface.php
├── Environment.php                 state and load(), using the traits
├── Traits/
│   ├── AppEnvTrait.php             getAppEnv(), isProduction(), isStaging(), isDevelopment(), isTesting(), isLocal()
│   ├── ValueTrait.php              getValue(), hasKey(), getRow()
│   └── EnvFileTrait.php            file lookup, parsing and updateRow()
├── Enums/, Exceptions/, Helpers/, Server.php

• EnvironmentInterface declares every public method of the class. setMutable() returns EnvironmentInterface in the contract, and Environment keeps : Environment, which narrows it.
• Environment keeps all its public methods, so environment(), env(), AppContext and the other users do not change.

Out of scope

• Server.
• The typed readers (getBool(), getInt(), env_bool(), env_int()): Add getBool() and getInt() to Environment, with env_bool() and env_int() helpers #615.
• Typed getters on Config: Add getBool() and getInt() to Config and use them instead of filter_var and (int) in the framework #616.

Breaking changes

• None.

Acceptance criteria

• EnvironmentInterface exists and Environment implements it
• the traits hold the code listed above, and Environment keeps only state and load()
• the existing environment tests pass without edits
• composer test, composer cs:check, composer phpstan and composer rector:check pass on every PHP version in the CI matrix
• the changelog lists the change