chore: add headers.Content-type to openFileInEditor call (#33517)

Summary:
Currently the `Content-type` is not defined and defaults to `text/plain`. This makes it more accurate.

## Changelog

<!-- Help reviewers and the release process by writing your own changelog entry. For an example, see:
https://github.com/facebook/react-native/wiki/Changelog
-->

[internal] [Changed] - add `headers.Content-type` to `openFileInEditor` call

Pull Request resolved: #33517

Test Plan: - [x] in development trigger `openFileInEditor`, e.g. by pressing on a line in a stack trace -> code editor opens

Reviewed By: cipolleschi

Differential Revision: D40022413

Pulled By: cortinico

fbshipit-source-id: 00ad3cdcdd1f82436c04aed3643c9f0b55778592

Author: andrejborstnik

Libraries/Core/Devtools/openFileInEditor.js

@@ -15,6 +15,9 @@ const getDevServer = require('./getDevServer');
 function openFileInEditor(file: string, lineNumber: number) {
   fetch(getDevServer().url + 'open-stack-frame', {
     method: 'POST',
+    headers: {
+      'Content-Type': 'application/json',
+    },
     body: JSON.stringify({file, lineNumber}),
   });
 }