.gitignore: ignore *.code-workspace files

those can be used in VSCode/VSCodium to save user-settings alongside the git-tracked .vscode/settings.json

inspired by https://laurens.io/blog/changing-vs-code-settings-locally-when-its-checked-into-git/

Author: mguetschow

.gitignore

@@ -39,6 +39,7 @@ cachegrind.out*
 # Visual Studio Code user settings
 .vscode/*
 !.vscode/settings.json
+*.code-workspace
 # ctags index files
 tags
 # GDB initialization scripts