Add vendor

Specifically because I want to modify a dependency

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

@@ -0,0 +1,9 @@
+//go:build js && !appengine
+// +build js,!appengine
+
+package runewidth
+
+func IsEastAsian() bool {
+	// TODO: Implement this for the web. Detect east asian in a compatible way, and return true.
+	return false
+}