Fix nvim-remote commands for fish shell

[SavingFrame]

Fish shell does not support "&&" and "||" operators like POSIX-compatible shells. Instead, it uses a different syntax structure based on begin/end and if/else.

This caused existing lazygit nvim-remote integration templates to break when fish was the user's default shell.

This commit adds explicit fish shell detection using the FISH_VERSION environment variable, and provides fish-compatible templates that correctly handle launching Neovim or sending remote commands via $NVIM.

Fixes behavior where edits would not open in a new Neovim tab or line navigation would fail when $NVIM was set.

Ensures smoother editing experience for users running fish shell (supported since Nov 2012 with FISH_VERSION).

• PR Description

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[stefanhaller]

Thanks for fixing this! It also came up here, and my suggestion for fixing it was to not use the users shell when calling editor commands, but always using bash as we did before. But your fix may be the better one, as it allows users to use aliases or custom paths for their editors, which came up as a request once or twice.

I pushed two fixups: one is to use same logic to decide which shell to use as the code that eventually calls the command (dc0e1a6); the other is to use more specific variable names (9a77469).

I'll let @jesseduffield decide if he's happy with the approach used.

[jesseduffield]

LGTM with the more specific variable names

[DerRockWolf]

This fix IMO isn't 100% ideal.

In my setup, I have custom nvim-remote commands (to not open the file in a new tab).

I can now update my lazygit config and fix the custom commands, but then my lazygit config is not shell agnostic anymore.
I would appreciate if the used shell (for the edit things) could be overridden somehow.