Finalize the isWritableFileSystem API

Fixes #91697

src/vs/vscode.d.ts

@@ -6860,6 +6860,21 @@ declare module 'vscode' {
 		 * @param options Defines if existing files should be overwritten.
 		 */
 		copy(source: Uri, target: Uri, options?: { overwrite?: boolean }): Thenable<void>;
+
+		/**
+		 * Check if a given file system supports writing files.
+		 *
+		 * Keep in mind that just because a file system supports writing, that does
+		 * not mean that writes will always succeed. There may be permissions issues
+		 * or other errors that prevent writing a file.
+		 *
+		 * @param scheme The scheme of the filesystem, for example `file` or `git`.
+		 *
+		 * @return `true` if the file system supports writing, `false` if it does not
+		 * support writing (i.e. it is readonly), and `undefined` if VS Code does not
+		 * know about the filesystem.
+		 */
+		isWritableFileSystem(scheme: string): boolean | undefined;
 	}
 
 	/**

src/vs/vscode.proposed.d.ts

@@ -2141,28 +2141,6 @@ declare module 'vscode' {
 	}
 	//#endregion
 
-	//#region https://github.com/microsoft/vscode/issues/91697
-
-	export interface FileSystem {
-		/**
-		 * Check if a given file system supports writing files.
-		 *
-		 * Keep in mind that just because a file system supports writing, that does
-		 * not mean that writes will always succeed. There may be permissions issues
-		 * or other errors that prevent writing a file.
-		 *
-		 * @param scheme The scheme of the filesystem, for example `file` or `git`.
-		 *
-		 * @return `true` if the file system supports writing, `false` if it does not
-		 * support writing (i.e. it is readonly), and `undefined` if VS Code does not
-		 * know about the filesystem.
-		 */
-		isWritableFileSystem(scheme: string): boolean | undefined;
-	}
-
-
-	//#endregion
-
 	//#region https://github.com/microsoft/vscode/issues/108929 FoldingRangeProvider.onDidChangeFoldingRanges @aeschli
 	export interface FoldingRangeProvider2 extends FoldingRangeProvider {