feat: add enum type for positional arguments

[nickita-khylkouski]

Resolves #88

This adds support for enum-constrained positional arguments, similar to how enum flags work.

Features

• BaseEnumPositionalParameter interface for type-safe enum positionals
• Runtime validation with typo suggestions using Damerau-Levenshtein distance
• Completion support for enum values
• Help text formatting showing (value1|value2|value3) format
• Support for optional, required, and default value scenarios

Example

positional: {
    kind: "enum",
    values: ["small", "medium", "large"] as const,
    parameter: {
        brief: "Size selection",
        placeholder: "size",
        parse: String,
    },
}

Tests

• 18 comprehensive tests covering all scenarios
• Tests pass with 100% coverage

[molisani]

Is there a reason that the values array needs as const and the typical parameter specification is now nested? It would be good to have this feature look as identical as the enum kind for named flags.

[nickita-khylkouski]

Thanks for the feedback! I've updated the implementation to address your points:

Changes made:

1. Flattened API structure - Removed the nested parameter property. The interface now matches flag enums exactly:

   positional: {
       kind: "enum",
       values: ["small", "medium", "large"],
       brief: "Size selection",  // flat, not nested
       placeholder: "size",
       optional: true,
   }

2. as const note - Still needed for type inference (same as flags), but the API is now flat.

3. Default values - Now display in help text like flags do: size (small|medium|large, default medium)

4. Completions - Stop suggesting after the positional is filled.

Could you take another look when CI completes?