implement lazyregexp package

Based on the "lazyregexp" package in golang.org/x/mod;
https://cs.opensource.google/go/x/mod/+/refs/tags/v0.19.0:internal/lazyregexp/lazyre.go;l=66-78

This package allows defining regular expressions that should not be
compiled until used, but still providing validation to prevent
invalid regular expressions from producing a panic at runtime.

This is largely a copy of the package from golang.org/x/mod,
with FindAllStringSubmatch and ReplaceAllStringFunc added

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

internal/lazyregexp/lazyregexp_test.go

@@ -0,0 +1,23 @@
+package lazyregexp
+
+import (
+	"testing"
+)
+
+func TestCompileOnce(t *testing.T) {
+	t.Run("invalid regexp", func(t *testing.T) {
+		defer func() {
+			if r := recover(); r == nil {
+				t.Errorf("expected a panic")
+			}
+		}()
+		_ = New("[")
+	})
+	t.Run("valid regexp", func(t *testing.T) {
+		re := New("[a-z]")
+		ok := re.MatchString("hello")
+		if !ok {
+			t.Errorf("expected a match")
+		}
+	})
+}