We have changed too much to consider it a mere bundled dependency (such
as unicode handling in e3bfcf2fd4a4ebf00b104b082cfe83c8144a842d), and
can consider it our own at this point.

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.