Preserve language picked for untitled text documents

[jrieken]

• create a new document
• select language to JavaScript
• type code
• exit and restart VS Code
• 🐛 the untitled file shows as Plaintext

[bpasero]

This is actually true for both files and untitled files. While this issue has been there for files already without hot-exit, it is a bit more obvious now for untitled files at least.

Related: #16588

[bpasero]

Actually fix was easy enough for untitled. Opened #17115 to track for files.