feature #35308 [Dotenv] Add Dotenv::bootEnv() to check for .env.local.php before calling Dotenv::loadEnv() (nicolas-grekas)

This PR was merged into the 5.1-dev branch.

Discussion
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[Dotenv] Add Dotenv::bootEnv() to check for .env.local.php before calling Dotenv::loadEnv()

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | yes
| Tickets       | -
| License       | MIT
| Doc PR        | -

The goal of this PR is to eventually get rid of the `config/bootstrap.php` file in Symfony 5.1 apps.
I think we've done enough iterations on that piece of bootstrapping logic to put it inside the `Dotenv` component.
This fully replaces https://github.com/symfony/recipes/blob/master/symfony/framework-bundle/4.2/config/bootstrap.php

It doesn't conflict with current apps so they'll be fine keeping the `config/bootstrap.php` file until they're upgraded.

The new bootstrapping logic will require adding this line in `bin/console` and `public/index.php`:
```php
(new Dotenv())->bootEnv(dirname(__DIR__).'/.env');
```

Recipes updated at symfony/recipes#724

Commits
-------

98c7d3027b [Dotenv] Add Dotenv::bootEnv() to check for .env.local.php before calling Dotenv::loadEnv()

Author: fabpot

composer.json

@@ -41,6 +41,7 @@
     },
     "conflict": {
         "symfony/dependency-injection": "<4.4",
+        "symfony/dotenv": "<5.1",
         "symfony/event-dispatcher": "<4.4",
         "symfony/lock": "<4.4",
         "symfony/process": "<4.4"