Added support for environment variables

[liuzicheng1987]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces support for reading from and writing to environment variables using rfl::env::read and rfl::env::write. The changes include the implementation of the Reader and Writer classes for environment variables, a new ToAllCaps case-transformation processor, comprehensive documentation, and a suite of unit tests. The review feedback highlights several critical issues that need to be addressed: portability concerns on Windows and macOS due to the direct use of POSIX-specific functions like setenv, unsetenv, and environ; missing locale headers for floating-point parsing; a bug in the substring length calculation when parsing environment variables; and a design flaw where empty environment variables are incorrectly treated as missing, which prevents empty strings from being parsed correctly.

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