feat(@effect/cli): add withOptionalValue combinator to Options

[Zelys-DFKH]

Closes #6182

The `--color[=WHEN]` / `--watch[=mode]` / argparse `nargs='?'` pattern: a flag that works bare with a sensible default, or accepts an explicit value. `@effect/cli` had no way to express this; any non-boolean option always required a trailing value.

`withOptionalValue(fallback)` fills that gap. The flag still must appear unless combined with `.optional()` or `.withDefault()`; only the value portion becomes optional:

```ts
const watch = Options.choice("watch", ["first-failure", "until-done"] as const).pipe(
Options.withOptionalValue("first-failure"),
Options.optional
)
// --watch → Some("first-failure")
// --watch until-done → Some("until-done")
// --watch=until-done → Some("until-done")
// (absent) → None
```

The implementation stays entirely inside `@effect/cli`. `Single` gains one field (`optionalValueDefault: Option`); the combinator is a standard `dual` that sets it via `modifySingle`. The three existing combinators that call `modifySingle` (`withAlias`, `withDescription`, `withPseudoName`) each thread the new field so chaining cannot silently clear it. The two "Expected a value following option" sites in `parseCommandLine` now emit an empty values array when `optionalValueDefault` is set, and `parseInternal` returns the fallback directly for an empty array. `getUsageInternal` wraps the type hint in brackets, so `--level []` appears in `--help` output.

12 new tests in `test/Options.test.ts` cover: supplied value, bare flag fallback, absent flag error, mid-args with and without a trailing value, composition with `optional()` and `withDefault()`, alias order independence (both orderings), `--flag=value` equals syntax, the canonical `choice` use case from the issue, and an integer option. 173 tests pass.

Thanks for the library. Happy to contribute.

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Latest commit: 6e7696b

The changes in this PR will be included in the next version bump.

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