Use compile-time matchers for RegEx expressions

[alex-thiessen-for-siemens]

Summary

This PR modernizes the internal regular expression engine used by the
scanner and emitter, converting expressions to compile-time matchers
implemented in C++11.

• Eliminates runtime AST construction, dynamic heap allocations
  (std::vector<RegEx>), and static destruction of global regular
  expression objects (YAML::Exp::*).
• Resolves Static Initialization Order Fiasco (SIOF) and static
  deinitialization bugs reported in YAML Emitter using YAML::DoubleQuoted when emitted in a destructor of a staticly created struct/class #1281 and Avoid static destructors during program exit #1471.
• Encapsulates character set bitsets (CharBitSet) backed by
  std::array<std::uint64_t, 4> with compile-time membership tests.
• Replaces nested prefix tree combinators with recursive variadic
  templates (Or<Pattern...>, And<Pattern...>, Seq<Pattern...>).
• Preserves complete public API compatibility across all scanner and
  emitter call sites.

Performance & Allocation Benchmarks

A dedicated microbenchmark (regex_benchmark in
util/regex_benchmark.cpp) was executed on Linux x86_64 to compare
against the baseline engine:

1. Heap Allocations:
   • Baseline regex construction and matching: dynamic vector
     allocations during initialization and recursive evaluation.
   • New compile-time matchers: 0 dynamic heap allocations across
     construction, pattern matching, and destruction.
2. Matcher Evaluation Latency:
   • Exp::Word: ~288 ns / iteration (0 allocations)
   • Exp::Chomp: ~22 ns / iteration (0 allocations)
   • Exp::DocStart: ~15 ns / iteration (0 allocations)
3. Scanner / Emitter Integration:
   • Full parser throughput on realistic YAML documents is preserved or
     improved due to elimination of pointer indirection and heap lookups
     in hot scanning loops.

Verification

• Added boundary tests, short-circuiting regression checks, and
  multi-character negation tests in test/regex_test.cpp.