chore(vscode): gitignore .vscode-test harness cache directory (#194)

The @vscode/test-electron harness downloads a full VS Code binary into
vscode-extension/.vscode-test/ (~900MB, 6300+ files), which flooded the
git working tree. Ignore it alongside the other build outputs.

Co-authored-by: Thomas Jung <12159356+jung-thomas@users.noreply.github.com>

Author: jung-thomas

vscode-extension/.gitignore

@@ -3,3 +3,4 @@ dist/
 webview-dist/
 _i18n/
 *.vsix
+.vscode-test/