build(deps): vendor libvterm at v0.3.3

Problem: Adding support for modern Nvim features (reflow, OSC 8, full
utf8/emoji support) requires coupling libvterm to Nvim internals
(e.g., utf8proc).

Solution: Vendor libvterm at v0.3.3.

src/clint.py

@@ -880,6 +880,7 @@ def CheckIncludes(filename, lines, error):
             "mpack/object.h",
             "nvim/func_attr.h",
             "termkey/termkey.h",
+            "vterm/vterm.h",
             ]
 
     for i in check_includes_ignore: