Fix VS Code extension

The VS Code extension is crashing on startup because
performance-writer.js is not included in the .vsix package. Include it,
fixing the crash.

Also include the performance writer instructions in case anyone wants to
play with it but doesn't have access to quick-lint-js' source
repository.

Author: strager

plugin/vscode/.vscodeignore

@@ -1,7 +1,9 @@
 **/*
 
 !LICENSE
+!PERFORMANCE-TRACING.md
 !README.md
 !dist/*
 !extension.js
 !package.json
+!performance-writer.js