Detect and warn when unsupported shells are being used (e.g. fish)

[sourishkrout]

Runme should be able to detect when an unsupported shell is being used via $SHELL and $0 and could warn accordingly.

The current error message looks something like this which isn't particularly helpful.

error: Internal failure executing runner: open /var/folders/hn/3tbqz8pd49q9yl2ns43qx6fh0000gp/T/512117330/.env_start: no such file or directory

Let's detect unsupported shells (e.g., fish, PowerShell).

Plan to show warning message with link to documentation for unsupported shells.

Provide button to open Documentation URL to be configurable for potential Stateful-specific overrides.

[sourishkrout]

Let's add a "Don't ask again" option to the notification, similar to https://github.com/stateful/vscode-runme/blob/main/src/extension/messaging.ts#L100-L102, @pastuxso, please. I think we want that before we can turn on the warning by default.

We should probably say Don't warn again.

[sourishkrout]

@pastuxso let's flip the experiment to be default true to expose it publicly.