feat: varargs .one_of() and .any_of(*literals) chain-level shorthand (#266)

Adds a varargs shorthand for literal alternation on both fluent surfaces
(`RegexBuilder` and `Pattern`):

- **`.any_of(*literals: str)`** — dual-mode overload. Zero args keeps
the existing frame-opener behavior
(`.any_of().string("cat").string("dog").end()`). One or more string args
skips the frame dance and appends the alternation directly.
- **`.one_of(*literals: str)`** — new method, always the varargs form.
Requires at least one literal (raises `MustBeAtLeastOneLiteralError` on
zero).

Literals go through the same escape + `CharElement`/`StringElement`
branching that `.char()` / `.string()` use, so the compile-path fusion
into `[...]` for single-character sets still applies:

```python
RegexBuilder().any_of("+", "-").to_regex_string()        # [\+\-]
RegexBuilder().one_of("cat", "dog", "fish").to_regex_string()  # (?:cat|dog|fish)
```

The pre-existing top-level factory `any_of(*operands: Pattern)` from
#263 stays untouched — it takes `Pattern` operands, not literal strings,
and serves the compositional (humre-style) API rather than the
chain-level shorthand this PR delivers.

Closes #131

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