Ensure Nushell is properly detected on Windows and Linux

[anthonykim1]

Nushell not detected on windows, linux.

[anthonykim1]

If nushell is installed on computer but nushell language extension is not installed, then user won't have nushell terminal profile contributed on their VS Code integrated terminal.

Feature: Perhaps recommend to install nushell extension if we detect nushell is installed on user computer.

Turns out Nushell is detected but there is some work to be done to make terminal state's shell type to be instant and
recommending nushell lang extension when we detect nushell installed on user's computer.

[anthonykim1]

Closing since we do detect shell type (including Nushell) on all platform.
Could improve experience with above comment though. Creating separate issues.

[anthonykim1]

Verification steps:

• Subscribe to onDidChangeTerminalState and specifically listen to shell property.
• Feel free to change between multiple shell and make sure shell events are fired accordingly.