feat(lazygit): allow user to override LazyVim config with custom file (#4367)

## Description
This takes into consideration an additional file, if it exists, to
extend `LG_CONFIG_FILE` environment variable, so that users can
overwrite easier default LazyVim options set for lazygit in
`lazygit-theme.yml`. It's not desirable to directly change
`lazygit-theme.yml` as the values are hardcoded and regenerated upon
colorscheme change.
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maintainers
  why we should accept this pull request. -->

## Related Issue(s)
Fixes #4364. Ideally, the problem about `nvim-remote` on Windows should
be fixed on lazygit's side, so this is just kind of a hotfix in the
meantime. Feel free to disregard this if not desirable.
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  If this PR fixes any issues, please link to the issue here.
  - Fixes #<issue_number>
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## Screenshots

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## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/config/options.lua

@@ -20,9 +20,12 @@ vim.g.root_spec = { "lsp", { ".git", "lua" }, "cwd" }
 
 -- LazyVim automatically configures lazygit:
 --  * theme, based on the active colorscheme.
---  * editorPreset to nvim-remote
+--  * editPreset to nvim-remote
 --  * enables nerd font icons
 -- Set to false to disable.
+-- Set the options you want to override in `~/.config/lazygit/custom.yml`
+-- WARN: on Windows you might want to set `editPreset: "nvim"` due to
+-- this issue https://github.com/jesseduffield/lazygit/issues/3467
 vim.g.lazygit_config = true
 
 -- Options for the LazyVim statuscolumn