Allow easier customization of the .env file

[TimothyBJacobs]

The .env file allows for configuring how the WordPress Local environment should be configured. However, because the file is version controlled, developers must be careful not to commit their modifications.

This commit renames the .env file to be .env.example. During env start, the .env.example file is copied to .env if it does not exist. This allows for contributors to continue using the project without thinking about .env and to make changes when needed. This brings WordPress Core into the dotenv project guidelines.

Props johnbillion, afragen, h71, desrosj.
Fixes #52668.

Trac ticket: https://core.trac.wordpress.org/ticket/52668

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Props timothyblynjacobs, johnbillion.

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[TimothyBJacobs]

Merged in r59038.