fix: add support for fileOpener in config.json (#911)

This PR introduces the following type:

```typescript
export type FileOpenerScheme = "vscode" | "cursor" | "windsurf";
```

and uses it as the new type for a `fileOpener` option in `config.json`.
If set, this will be used to linkify file annotations in the output
using the URI-based file opener supported in VS Code-based IDEs.

Currently, this does not pass:

Updated `codex-cli/tests/markdown.test.tsx` to verify the new behavior.
Note it required mocking `supports-hyperlinks` and temporarily modifying
`chalk.level` to yield the desired output.

codex-cli/tests/terminal-chat-response-item.test.tsx

@@ -38,7 +38,10 @@ function assistantMessage(text: string) {
 describe("TerminalChatResponseItem", () => {
   it("renders a user message", () => {
     const { lastFrameStripped } = renderTui(
-      <TerminalChatResponseItem item={userMessage("Hello world")} />,
+      <TerminalChatResponseItem
+        item={userMessage("Hello world")}
+        fileOpener={undefined}
+      />,
     );
 
     const frame = lastFrameStripped();
@@ -48,7 +51,10 @@ describe("TerminalChatResponseItem", () => {
 
   it("renders an assistant message", () => {
     const { lastFrameStripped } = renderTui(
-      <TerminalChatResponseItem item={assistantMessage("Sure thing")} />,
+      <TerminalChatResponseItem
+        item={assistantMessage("Sure thing")}
+        fileOpener={undefined}
+      />,
     );
 
     const frame = lastFrameStripped();