Add Vietnamese support to DejaVu Sans Code

[tduyduc]

Vietnamese language support has long been a big flaw in the DejaVu Sans Mono family. I created a PR request in the base DejaVu Fonts repository, but I've got no response since then.

In this PR, I add missing glyphs in U+1EA0 – U+1EF9 range. This should complete the Vietnamese language cover (when langcover.txt is updated). If not, please let me know, I'll find missing glyphs to add them.