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_windows.go

@@ -0,0 +1,24 @@
+package runewidth
+
+import (
+	"syscall"
+)
+
+var (
+	kernel32               = syscall.NewLazyDLL("kernel32")
+	procGetConsoleOutputCP = kernel32.NewProc("GetConsoleOutputCP")
+)
+
+func IsEastAsian() bool {
+	r1, _, _ := procGetConsoleOutputCP.Call()
+	if r1 == 0 {
+		return false
+	}
+
+	switch int(r1) {
+	case 932, 51932, 936, 949, 950:
+		return true
+	}
+
+	return false
+}