improve shell identify on case-insensitive system

Use case-insensitive regex to remove `.exe`  extension.

microsoft#22036 (comment)

src/client/common/terminal/shellDetectors/baseShellDetector.ts

@@ -63,7 +63,7 @@ export abstract class BaseShellDetector implements IShellDetector {
 export function identifyShellFromShellPath(shellPath: string): TerminalShellType {
     // Remove .exe extension so shells can be more consistently detected
     // on Windows (including Cygwin).
-    const basePath = shellPath.replace(/\.exe$/, '');
+    const basePath = shellPath.replace(/\.exe$/i, '');
 
     const shell = Array.from(detectableShells.keys()).reduce((matchedShell, shellToDetect) => {
         if (matchedShell === TerminalShellType.other) {