feat(c): rename DecodeFunction to TSDecodeFunction

Keep a typedef for backwards compatibility until ABI 16.

lib/src/lexer.c

@@ -114,7 +114,7 @@ static void ts_lexer__get_lookahead(Lexer *self) {
   }
 
   const uint8_t *chunk = (const uint8_t *)self->chunk + position_in_chunk;
-  DecodeFunction decode =
+  TSDecodeFunction decode =
     self->input.encoding == TSInputEncodingUTF8    ? ts_decode_utf8     :
     self->input.encoding == TSInputEncodingUTF16LE ? ts_decode_utf16_le :
     self->input.encoding == TSInputEncodingUTF16BE ? ts_decode_utf16_be : self->input.decode;