chore: do not carry what is not the package

Two files were tracked that describe a machine rather than the package: one
editor's settings, and a compilation database whose every entry is an absolute
path on the machine that produced it. Neither is reproducible, neither is read by
anything that builds this package, and both change whenever somebody opens the
directory.

The ignore list is now the same in all six repositories of this project, and it
names what a build produces, what an editor leaves beside the sources, and what a
file browser leaves behind. A repository that carries one editor's settings has
an opinion about editors.

Author: Sunrisepeak

.gitignore

@@ -1,4 +1,19 @@
+# What a build produces.
 target/
 .mcpp/
-compile_commands.json
 mcpp.lock
+compile_commands.json
+
+# What an editor or a tool leaves beside the sources. None of it describes the
+# package: a repository that carries one editor's settings has an opinion about
+# editors, and a repository that carries a compilation database has an opinion
+# about one machine's paths.
+.vscode/
+.idea/
+.cache/
+*.swp
+*~
+
+# What a system leaves behind.
+.DS_Store
+Thumbs.db