gitignore: Ignore core dumps

[chrysn]

Contribution description

#18541 illustrates that core dumps can get in by accident. As these are a similar category as built binaries,

This excludes all files (but not directories) named core

Testing

• Touch a file core; it will not show in git status.
• Touch a file named differently inside core/ (or a newly created directory of that name); it will show.

[chrysn]

Nevermind, that was done concurrently in #18541 -- in a different way, but equally effective.