Add explicit option enum value __MAX_VALUE (#7547)

Add explicit option enum value __MAX_VALUE (#7547)

This PR fixes a subtle issue with the dict ordering changing across different python versions.
To store the flags of -o, we store a bitset with the index being the enum value cast to size_t. To specify the bitset size, we simply used the last enum value (in the toml file). This, however, assumes that the order within dictionaries is stable: toml simply puts all modes in a python dict.
This PR avoids this issue by introducing an explicit __MAX_VALUE enum value. Note that this also avoids the need to update the bitset definition when new output tags are added.