vendor: only vendor on emitted binaries

Moves the vendor/ directory to cmd/vendor. Vendored binaries are built
from cmd/, which is backed by symlinks pointing back to repo root.

cmd/vendor/github.com/mattn/go-runewidth/runewidth_js.go

@@ -0,0 +1,8 @@
+// +build js
+
+package runewidth
+
+func IsEastAsian() bool {
+	// TODO: Implement this for the web. Detect east asian in a compatible way, and return true.
+	return false
+}