Handle having multiple Swift toolchains on Windows (#795)

It is possible for `which swift` to return multiple lines if the user
has more than one Swift toolchain installed. Windows behavior is to use
the first one, so we use it as well but also warn the user.

Author: plemarquand

src/toolchain/toolchain.ts

@@ -315,7 +315,13 @@ export class SwiftToolchain {
                     }
                     case "win32": {
                         const { stdout } = await execFile("where", ["swift"]);
-                        swift = stdout.trimEnd();
+                        const paths = stdout.trimEnd().split("\r\n");
+                        if (paths.length > 1) {
+                            vscode.window.showWarningMessage(
+                                `Found multiple swift executables in in %PATH%. Using excutable found at ${paths[0]}`
+                            );
+                        }
+                        swift = paths[0];
                         break;
                     }
                     default: {