chore: gitignore files with an extension *.db

For example the commmand `cargo run --package limbo_cli --bin limbo database.db`
will generate a `database.db` file, and it's unlikely we'll ever need to
track such files if they are placed inside the root of the project.

.gitignore

@@ -20,6 +20,7 @@ dist/
 .tmp/
 
 
+*.db
 **/*.db-wal
 **/*.db-shm
 **/*-wal
@@ -32,4 +33,4 @@ dist/
 
 # testing
 testing/limbo_output.txt
-**/limbo_output.txt
+**/limbo_output.txt