Add vendor

Specifically because I want to modify a dependency

vendor/github.com/mattn/go-runewidth/runewidth_windows.go

@@ -0,0 +1,28 @@
+//go:build windows && !appengine
+// +build windows,!appengine
+
+package runewidth
+
+import (
+	"syscall"
+)
+
+var (
+	kernel32               = syscall.NewLazyDLL("kernel32")
+	procGetConsoleOutputCP = kernel32.NewProc("GetConsoleOutputCP")
+)
+
+// IsEastAsian return true if the current locale is CJK
+func IsEastAsian() bool {
+	r1, _, _ := procGetConsoleOutputCP.Call()
+	if r1 == 0 {
+		return false
+	}
+
+	switch int(r1) {
+	case 932, 51932, 936, 949, 950:
+		return true
+	}
+
+	return false
+}