Fixes: #10828

For consideration. Telemetry suggests that less than 3% of workspaces have more than 100,000 source files (with 250,000 total files). That seems like a reasonable threshold, but we could raise it if we think that's too many customers to notify.

The notification can be disabled for the workspace. The "learn more" button links to a page I wrote in the Wiki about improving the configuration (which should also be reviewed). https://github.com/microsoft/vscode-cpptools/wiki/Optimizing-your-configuration