internal/format: require the last line of stanzas to be short

We are going to reuse the stanza format for IPC in the plugin protocol,
but in that context we need stanzas to be self-closing. Currently they
almost are, but if the body is 0 modulo 48, there is no way to know if
the stanza is over after the last line.

Now, all stanzas have to end with a short line, even if empty.

No ciphertexts generated by age in the past are affected, but 3% of the
ciphertexts generated by rage will now stop working. They are still
supported by rage going forward. If it turns out to be a common issue,
we can add an exception.

cmd/age/testdata/empty_recipient_body.age

@@ -1,6 +1,7 @@
 age-encryption.org/v1
--> ssh-ed25519 o1Hudg SZISkI5Qn8YgUBmTKG/Zp/QpFjXWvAivzvB+hOcN5W8
-dYfwGWYvCwpSU5EXIC1XqfXdsBvCi3kMypdqCVShrpk
--> joint-oil-hw
---- gC/27VAgqOEzAQMKHvBjih7sJ1oDKht+HNdguTIbjt8
-f<EFBFBD>tAe<EFBFBD>֨&8{<7B><><EFBFBD>νcat<61><1B><><16><><EFBFBD><13>˷}<17>=<3D>C<EFBFBD><43>u
+-> X25519 alRneDshIh43nwyD5+fhuTD5TReSn88f2us4hzZPyzU
+pGduNK5MUhnuzMxW0qbZnC2k7mRzz69bbJpKQrRc7uc
+-> A7)h-grease !,_
+
+--- 5bA0uXjBxI6wuI5SseCRgD5/G8LkSVISRe/hnrQMb9s
+<EFBFBD><EFBFBD><06>1<EFBFBD><31><EFBFBD><EFBFBD><03>6_R<5F><08>څ<EFBFBD><DA85>U<<3C>1<EFBFBD>s<EFBFBD><73>?`<60>+<><7F>$<24>H<EFBFBD>W<EFBFBD>v?w8ZW

cmd/age/testdata/empty_recipient_body_key.txt

@@ -1,7 +1 @@
------BEGIN OPENSSH PRIVATE KEY-----
-b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
-QyNTUxOQAAACAwKgrb/LkvtI887QylSoUh5xUlKr1fb37euR6et5jHowAAAJgxqUx+MalM
-fgAAAAtzc2gtZWQyNTUxOQAAACAwKgrb/LkvtI887QylSoUh5xUlKr1fb37euR6et5jHow
-AAAEC7gKj74YIwaM1BT2tnODjfeZJvo8lcazvL6Uljv3+nIDAqCtv8uS+0jzztDKVKhSHn
-FSUqvV9vft65Hp63mMejAAAADnJ1bm5lckBmdi1hejMyAQIDBAUGBw==
------END OPENSSH PRIVATE KEY-----
+AGE-SECRET-KEY-1TRYTV7PQS5XPUYSTAQZCD7DQCWC7Q77YJD7UVFJRMW4J82Q6930QS70MRX