Disable formatter in src/lib/* (#138)

Otherwise VS Code reformats the whole file while working on submodules
(if "Format on save" is activated). The submodules aren't prepared for
this yet (no "format all files" commit) and should include their own
definitions anyway, especially since they could be third-party with
different ideas of code style.

The solution was taken from
https://stackoverflow.com/a/57272592/25601664

I also tried creating a `.clang-format-ignore` file with content
`src/lib/*`, but couldn't get it working.

src/lib/.clang-format

@@ -0,0 +1,2 @@
+DisableFormat: true
+SortIncludes: Never