Allow preserving existing replacement characters

[agammann]

Addresses the separator collision reported in #141.

Add an opt-in preserveReplacement option so characters already equal to the chosen replacement are kept instead of being folded into surrounding whitespace. This makes step -1 become step--1 while step 1 remains step-1, and keeps 1952-- intact. The default behavior is unchanged. Document the option and add its TypeScript declaration and regression cases. This is separate from #179, which proposes preserving arbitrary characters through strict filtering.

Baseline reproduction: both step -1 and step 1 produced step-1; 1952-- produced 1952. Local verification: direct Mocha run passed all 46 tests; direct Node output confirmed the three expected cases and unchanged default; a TypeScript import using the new option compiled with tsc --noEmit; git diff --cached --check passed.