Modernize type hints and dispatchers for Python 3.10+

[Copilot]

Since pySigma requires Python 3.10+, modernize the codebase to use contemporary syntax that was previously unavailable.

Type annotations → PEP 604 union syntax

• Union[X, Y] → X | Y (~170 occurrences)
• Optional[X] → X | None (~156 occurrences)
• Dict, List, Set, Pattern → dict, list, set, re.Pattern
• Removed now-unused typing imports

# Before
def foo(val: Optional[Union[str, int]] = None) -> Union[str, None]:
# After
def foo(val: str | int | None = None) -> str | None:

Condition dispatchers → match/case

Converted the three isinstance dispatch chains in conversion/base.py (convert_condition_field_eq_val, convert_condition_val, convert_condition) to structural pattern matching.

# Before
if isinstance(cond.value, SigmaCasedString):
    return self.convert_condition_field_eq_val_str_case_sensitive(cond, state)
elif isinstance(cond.value, SigmaString):
    ...

# After
match cond.value:
    case SigmaCasedString():
        return self.convert_condition_field_eq_val_str_case_sensitive(cond, state)
    case SigmaString():
        ...

Runtime type inspection compatibility

X | Y produces types.UnionType at runtime, not typing.Union. Updated the two places that inspect types at runtime:

• modifiers.py: type_check() now handles both Union and UnionType
• transformations/base.py: Switched from __annotations__ to get_type_hints() (required with from __future__ import annotations) and added UnionType handling