shell: separate posix-shells from bash

[Noodlez1232]

Description

Creates a posix-compliant .profile and .shinit usable from any posix-compliant shell. This is great for runtime scripts to have their variables set without having to import programs.enable.bash

For backwards compatibility, it is enabled by default only if programs.enable.bash is enabled.

This commit also changes bash's .profile to be .bash_profile, once again, to keep them separate, since bash is not posix compliant, its profile should not be included in the normal .profile.

fixes: #5474
fixes: #6412

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt-rfc-style keep-sorted --run treefmt.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

• Test cases updated/added. See example.

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  {component}: {description}
  
  {long description}
  

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• If this PR adds a new module

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Maintainer CC

@rycee

[Noodlez1232]

This should be ready to go, but there is a couple questions I have about it:

1. Is the import of .bashrc required in .bash_profile
2. Should this be default enabled for stateVersion 25.11? Right now it's only enabled by default if bash is enabled (for backwards compatibility)

[Noodlez1232]

Oh and also I did test the POSIX side of it with two different shells:

1. Bash in posix mode (invoked using sh)
2. Dash, since it itself is a posix compliant shell.

And it worked perfectly for me without breaking actual bash support.