Add Avro Phonetic Bengali input method

[nahidhasanriad]

Summary

Adds Avro Phonetic — a popular phonetic transliteration input method for typing Bengali (Bangla) using the Latin alphabet. This implementation is a faithful port of the OmicronLab Avro Phonetic algorithm, which is widely used in Bangladesh and West Bengal.

What's included

• AvroPhoneticCombiner / AvroPhoneticEngine — core conversion engine implementing the longest-pattern-match algorithm with context-sensitive rules (prefix/suffix vowel, consonant, punctuation checks)
• avro_phonetic.json — the full OmicronLab Avro Phonetic specification (~1700+ pattern mappings with rules)
• functional_keys_avro.json — bottom-row key layout tailored for Avro typing
• Wiring in CombinerChain.kt to activate the combiner when combiningSpec == "bn_avro"
• Case-sensitive handling for characters that have distinct uppercase/lowercase meanings (a/A, o/O, i/I, u/U, d/D, g/G, j/J, n/N, r/R, s/S, t/T, y/Y, z/Z)

Usage

1. Enable Bengali Avro in keyboard settings
2. Type phonetically using Latin letters (e.g., hyalO → হ্যালো, Bangla → বাংলা, ami → আমি)
3. Use Shift for case-sensitive vowel/consonant variants (e.g., O for ◌ো vs o for dead consonant, D for ড vs d for দ)