Copy go-bip39 into internal/bip39 (closes #42)

go-bip39 is gone from GitHub, so keyfunc now carries the part of
v1.1.0 it uses, with the upstream LICENSE beside it, and imports it
from internal/bip39. Upstream's tests and test vectors for the kept
code come along unchanged.

Changes beyond the trimming are what the linter asked for: the
package-level variables moved into the functions that use them, three
error strings were lower-cased, and the unknown-word error now wraps
ErrInvalidMnemonic.

go.mod no longer requires go-bip39. go mod tidy keeps its two go.sum
lines, because a test in sneak/secret's bip85 package imports it, and
drops five lines that only go-bip39's own requirements needed.

Model: opus-5-5

internal/derive/derive.go

@@ -8,7 +8,7 @@ import (
 	"git.eeqj.de/sneak/secret/pkg/bip85"
 	"github.com/btcsuite/btcd/btcutil/hdkeychain"
 	"github.com/btcsuite/btcd/chaincfg"
-	bip39 "github.com/tyler-smith/go-bip39"
+	"sneak.berlin/go/keyfunc/internal/bip39"
 )
 
 const (