VS Code Intellisense broken on new files

[Rheeseyb]

• Either open an existing project or create a new one
• Open any file from that project, and start typing code that would offer suggestions to autocomplete and import a symbol from another file (so e.g. const a = FlexR in the default project would offer FlexRow from /src/utils.js). Witness that this works
• Create a new source file anywhere in that project
• Start typing the above code in the new source file. Witness that there are no suggestions from the current project.
• In the new source file, manually add an import statement for anything from another existing file
• Now you will start seeing suggestions for other symbols exported from that other existing file, but nowhere else in the project
• Reload the project and you'll now see suggestions from the rest of the project when editing the new file

[Rheeseyb]

I've spent quite a few hours now debugging this, and have failed to find anything of use here. The process we follow when adding an arbitrary new file is the exact same as when first opening the project in the first place (see INIT_PROJECT and WRITE_PROJECT_FILE in handleFromUtopiaToVSCodeMessage. I have verified that the issue only affects a file system coming from a file system provider (i.e. an in-memory file system), and I suspect that the issue lies somewhere inside either the Typescript Language Features plugin, or in the TS server itself.