Fix GitHub language statistics

[troeger]

Currently, our project is marked as JavaScript project, because of the large amount of third party code in this language.

Since GitHub intentionally does not support repo-specific exclusion lists, we could adopt our third-party code directory names according to their blacklist:

https://github.com/github/linguist/blob/master/lib/linguist/vendor.yml

[rakhimov]

Separating the third-party code (dependencies) into git submodules can help.