Motivation

To provide robust usage upon shell type context, only set shell-type context key if we can tell the type of the shell.

Also update the description of the context to reflect this change.

This modification resolve #191650.

Since that the semantic of the context key is also changed, the translation should also be updated.
But I'm not sure about the work flow regarding this.

How to Test

(This PR can only be verified since that terminalShellType context key is only supported when the host environment is Windows.)

Prepare a shell profile that the shell type is undefined.

--- a/src/vs/platform/terminal/node/windowsShellHelper.ts
+++ b/src/vs/platform/terminal/node/windowsShellHelper.ts
@@ -136,7 +136,7 @@ export class WindowsShellHelper extends Disposable implements IWindowsShellHelpe
        getShellType(executable: string): TerminalShellType | undefined {
                switch (executable.toLowerCase()) {
                        case 'cmd.exe':
-                               return WindowsShellType.CommandPrompt;
+                               return undefined;
                        case 'powershell.exe':
                        case 'pwsh.exe':
                                return WindowsShellType.PowerShell;

Open a shell, inspect the context key terminalShellType is set correctly.

Open another shell that do not support shell-type,
inspect again to check that the context key terminalShellType is disappeared.

(Also change between two opened shells to validate the behavior)