fix: op_codeseparator in tapscripts

[sander2]

Fixes script evaluation for taproot scripts that contain op_codeseparator. Before this pr, opcode_count was not incremented for taproot scripts. On top of that, the behavior of op_codeseparator changed in BIP342 compared to non-taproot scripts:

codesep_pos (4): the opcode position of the last executed OP_CODESEPARATOR before the currently executed signature opcode, with the value in little endian (or 0xffffffff if none executed). The first opcode in a script has a position of 0. A multi-byte push opcode is counted as one opcode, regardless of the size of data being pushed. Opcodes in parsed but unexecuted branches count towards this value as well.