This largely copies over Spicy's `.clang-format` configuration file. The
one place where we deviate is header include order since Zeek depends on
headers being included in a certain order.
This enables locating the headers within the install-tree using the
dirs provided by `zeek-config --include_dir`.
To enable locating these headers within the build-tree, this change also
creates a 'build/src/include/zeek -> ..' symlink.