Dotenv should gracefully handle or ignore shell-escaped and multiline environment values

[mhkarimi1383]

In CI environments it is very common for environment variables to contain:

• newlines
• shell-escaped values (e.g. $'...\n')
• injected metadata such as commit messages

When generating a .env file from the current process environment (for example
using declare -px or similar mechanisms), PHP dotenv fails hard with:

Failed to parse dotenv file.
Encountered unexpected whitespace at [$'Merge branch \'test\'\n'].

This behavior is problematic because:

• The environment itself is valid and usable by the shell
• Other ecosystem tools (Docker, systemd, Node dotenv, Go env loaders, etc.)
  either ignore unsupported entries or treat them as literal strings
• Dotenv crashes the entire application instead of handling the case defensively

Expected behavior:

• Unsupported or non-dotenv-compatible variables should be skipped or sanitized
• Multiline or shell-escaped values should not cause a fatal parsing error
• At minimum, dotenv should fail gracefully and allow applications to decide how
  to proceed

Current behavior makes CI pipelines fragile, since injected environment variables
outside of the application's control can break builds unexpectedly.

This is especially painful during Composer scripts (e.g. artisan package:discover)
where there is no reasonable recovery path.

Handling this defensively would align dotenv with how other environment loaders
behave in production and CI systems.