neovimUtils.wrapNeovimUnstable: support vim.o.exrc (#395991)

- `neovimRcContent`: Extra vimL code sourced by the generated `init.lua`.

- `wrapperArgs`: Extra arguments forwarded to the `makeWrapper` call.

- `wrapRc`: Nix, not being able to write in your `$HOME`, loads the

  generated Neovim configuration via its  `-u` argument, i.e. : `-u /nix/store/...generatedInit.lua`. This has side effects like preventing Neovim from reading your config in `$XDG_CONFIG_HOME` (see bullet 7 of [`:help startup`](https://neovim.io/doc/user/starting.html#_initialization) in Neovim). Disable it if you want to generate your own wrapper. You can still reuse while reusing the logic of the nixpkgs wrapper and access the generated config via `neovim.passthru.initRc`.

  generated Neovim configuration via the `$VIMINIT` environment variable, i.e. : `export VIMINIT='lua dofile("/nix/store/…-init.lua")'`. This has side effects like preventing Neovim from sourcing your `init.lua` in `$XDG_CONFIG_HOME/nvim` (see bullet 7 of [`:help startup`](https://neovim.io/doc/user/starting.html#startup) in Neovim). Disable it if you want to generate your own wrapper. You can still reuse the generated vimscript init code via `neovim.passthru.initRc`.

- `plugins`: A list of plugins to add to the wrapper.

```

### Additions and Improvements {#sec-nixpkgs-release-25.11-lib-additions-improvements}

- Create the first release note entry in this section!

- `neovim`: Added support for the `vim.o.exrc` option, the `VIMINIT` environment variable, and sourcing of `sysinit.vim`.

  See the neovim help page [`:help startup`](https://neovim.io/doc/user/starting.html#startup) for more information, as well as [the nixpkgs neovim wrapper documentation](#neovim-custom-configuration).

      viAlias ? false,

      # additional argument not generated by makeNeovimConfig

      # it will append "-u <customRc>" to the wrapped arguments

      # it sets the VIMINIT environment variable to "lua dofile('${customRc}')"

      # set to false if you want to control where to save the generated config

      # (e.g., in ~/.config/init.vim or project/.nvimrc)

      wrapRc ? true,

            "${placeholder "out"}/rplugin.vim"

          ]

          ++ lib.optionals finalAttrs.wrapRc [

            "--add-flags"

            "-u ${writeText "init.lua" rcContent}"

            "--set-default"

            "VIMINIT"

            "lua dofile('${writeText "init.lua" rcContent}')"

          ]

          ++ finalAttrs.generatedWrapperArgs;