feat(cli): add --stdin support for piping env content (#808)

Closes #808

This change introduces two new flags to dotenv-linter. The --stdin flag allows reading .env content from standard input, while --stdin-filename lets users specify a filename for diagnostic output when piping. In src/cli.rs, build_cli() has been updated to declare these options. In src/lib.rs, the run() function now checks for --stdin and either reads from stdin or from the provided file paths accordingly. Finally, an integration test named stdin_flag_works was added in tests/cli.rs to verify proper handling of piped input.
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Commit messages have been written in Conventional Commits format

Tests for the changes have been added (tests/cli.rs)

Documentation on dotenv-linter.github.io has been updated (if applicable)