add vendoring

vendor/github.com/jhillyerd/enmime/internal/stringutil/find.go

@@ -0,0 +1,39 @@
+package stringutil
+
+// FindUnquoted returns the indexes of the instance of v in s, or empty slice if v is not present in s.
+// It ignores v present inside quoted runs.
+func FindUnquoted(s string, v rune, quote rune) []int {
+	escaped := false
+	quoted := false
+	indexes := make([]int, 0)
+	quotedIndexes := make([]int, 0)
+
+	for i := 0; i < len(s); i++ {
+		switch rune(s[i]) {
+		case escape:
+			escaped = !escaped // escape can escape itself.
+		case quote:
+			if escaped {
+				escaped = false
+				continue
+			}
+
+			quoted = !quoted
+			if !quoted {
+				quotedIndexes = quotedIndexes[:0] // drop possible indices inside quoted segment
+			}
+		case v:
+			escaped = false
+			if quoted {
+				quotedIndexes = append(quotedIndexes, i)
+			} else {
+				indexes = append(indexes, i)
+			}
+		default:
+			escaped = false
+		}
+
+	}
+
+	return append(indexes, quotedIndexes...)
+}