Add LeadingStrings benchmarks for binary and non-ASCII regex patterns

[danmoseley]

Adds two new benchmark classes to exercise the LeadingStrings vs FixedDistanceSets heuristic in the regex engine:

• Perf_Regex_LeadingStrings_BinaryData: 1MB binary corpus (PE-header-like seed duplicated), alternation of binary patterns. Validates no regression on non-text input.
• Perf_Regex_LeadingStrings_NonAscii: ~100KB Russian text (Anna Karenina opening), alternation of Russian words. Validates no regression on non-ASCII text where the frequency heuristic bails out.

Companion to dotnet/runtime change: danmoseley/runtime#31