Allow case-sensitive environment variable setting

Basics

• I have done a basic search through the issue tracker to find similar or related issues.
• I have made myself familiar with the available features of Nushell for the particular area this enhancement request touches.

Related problem

Nushell treats all environment variables as case-sensitive, and has #17558 made this more consistent on all platforms.
Unfortunately, as #14551 correctly notes, this goes against the Unix convention and POSIX (https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html).

As also noted in that issue, this becomes particularly problematic when multiple variables that only differ in casing are present. Personally, it caused me a decent amount of headache just now to figure out why $env.SHELL is suddenly modified when loading a Nix devShell using direnv.

Describe the solution you'd like

It would therefore be great to have a way of defining/assigning env vars in a case-respecting manner. That means at minimum not coercing an assignment to shell into an assignment to the existing variable SHELL, and ideally allowing two variables that only differ in casing (such as SHELL and shell) to coexist.

It might be implemented in the form of a special flag for load-env and similar commands.

Describe alternatives you've considered

Ideally, I would love to be able to turn off case-insensitivity entirely, since I see little benefit (beyond Windows compatibility, which is not relevant for my use case) and believe it only causes more issues and annoying edge cases.

But as Nushell seems to have committed to case insensitivity I assume such a toggle would be difficult to realize and would probably cause a bunch of incompatibility between scripts etc.

Additional context and details

No response

Start by tracing Nushell's environment-variable assignment and loading behavior, especially the paths used by load-env and related commands. Define how case-respecting assignment and coexistence of differently cased variables should work, then add coverage for the selected behavior and verify it on the relevant platforms.