core: don't force shell integration unless we're running that shell

[jcollie]

If you bypassed shell detection and forced a specific shell integration with shell-integration=<shell> the bash and nushell integrations would break commands that were specified with +new-window -e <command> (or a similar mechanism) because they modify the command in shell-specific ways and the new command would fail with an "unknown flag" error or similar.

This PR fixes that by only permitting shell integration if the command matches the type of shell integration. That means that shell-integration=nushell only works if argv[0] ends with nu, etc. Generally this should not matter unless the shell executable was renamed for some reason and the shell detection failed.

Fixes #12378

[jparise]

I can't look in detail right now (temporarily internet-less), but my initial impression is that this approach isn't quite right.

• The purpose of "forced" shell integration is to ignore the command-based detection, generally for cases when the command is named something else (for reasons we don't know or necessarily need to understand). With the proposed approach, we're basically just using a hinted "detect" path.
• If we don't want shell integration in the +new-window path, I think we should skip calling shell_integration.setup() or pass a somehow different command into it. (This is the part I can't research while on mobile, so sorry if I'm missing something.)