lnwallet: check mempool acceptance before publishing

This commit adds a mempool acceptance check before broadcasting a given
transaction. To maintain the current behavior from
`BtcWallet.PublishTransaction`, the two errors, `ErrInMempool` and
`ErrAlreadyConfirmed` returned from `TestMempoolAccept` are ignored.

lnutils/errors.go

@@ -0,0 +1,21 @@
+package lnutils
+
+import "errors"
+
+// ErrorAs behaves the same as `errors.As` except there's no need to declare
+// the target error as a variable first.
+// Instead of writing:
+//
+//	var targetErr *TargetErr
+//	errors.As(err, &targetErr)
+//
+// We can write:
+//
+//	lnutils.ErrorAs[*TargetErr](err)
+//
+// To save us from declaring the target error variable.
+func ErrorAs[Target error](err error) bool {
+	var targetErr Target
+
+	return errors.As(err, &targetErr)
+}