umqtt.simple: Encode subscribe Remaining Length as VBI.

[pablogventura]

Summary

Fixes #969. subscribe (and unsubscribe) packed Remaining Length with
struct.pack_into(\"!BH\", ...), so values above 127 overflow a single byte
and long topics fail on the wire.

MQTT 3.1.1 section 2.2.3 requires a Variable Byte Integer. publish already
encodes Remaining Length that way; this PR uses the same pattern for
subscribe and unsubscribe without extracting a shared helper or changing
connect / publish.

Testing

• Unix port: micropython micropython/umqtt.simple/test_umqtt_simple.py
  (mock socket, no broker):
  • short topic (single-byte Remaining Length)
  • subscribe with topic length 123 (Remaining Length 128 -> 82 80 01)
  • unsubscribe with topic length 124 (Remaining Length 128 -> a2 80 01)
• simple.mpy via mpy-cross vs upstream/master: 2524 -> 2689 bytes (+165).

Trade-offs and Alternatives

• The VBI loop is duplicated from publish rather than factored out, to keep
  the diff small and avoid touching working paths.
• unsubscribe is included because it had the same one-byte Remaining Length
  bug.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.