Set plain text language mode when saving as plain text

<!-- ⚠️⚠️ Do Not Delete This! bug_report_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- 🕮 Read our guide about submitting issues: https://github.com/microsoft/vscode/wiki/Submitting-Bugs-and-Suggestions --> <!-- 🔎 Search existing issues to avoid creating duplicates. --> <!-- 🧪 Test using the latest Insiders build to see if your issue has already been fixed: https://code.visualstudio.com/insiders/ --> <!-- 💡 Instead of creating your report here, use 'Report Issue' from the 'Help' menu in VS Code to pre-fill useful information. --> <!-- 🔧 Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: Yes <!-- 🪓 If you answered No above, use 'Help: Start Extension Bisect' from Command Palette to try to identify the cause. --> <!-- 📣 Issues caused by an extension need to be reported directly to the extension publisher. The 'Help > Report Issue' dialog can assist with this. --> - VS Code Version: 1.74.1 - OS Version: Windows 11 Steps to Reproduce: 1. Set the files.defaultLanguage setting to ${activeEditorLanguage}  2. Open a JavaScript file 3. Open a new file with the command: workbench.action.files.newUntitledFile 4. Save the file with a .txt file extension 5. Notice that the file still has javascript syntax highlighting and is still formatted as a JavaScript file. If I use the command 'Change Language Mode' and then specify the language mode as a plain text file, the JavaScript wiggly underlines go away and the JavaScript syntax highlighting goes away, but it's annoying to have to do this every time I create a new plain text file.