Change defaultValue type to unknown

option and requiredOption's defaultValue parameter was `string |
boolean` but the default for a variadic option should be an array.

```
program.option('--var <args...>', 'variadic arguments', ['1'])
program.parse([])
> { var: ['1'] }

program.parse(['--var', '1', '2'])
> { var: ['1', '2'] }
```

If you use a string default you have to handle opts that are strings
```
// with a string arg the default value is a string.
program.option('--var <args...>', 'variadic arguments', '1')
program.parse([])
> { var: '1' }

program.parse(['--var', '1', '2'])
> { var: ['1', '2'] }
```

I chose unknown because it matches the jsdoc comments `@param {*}
defaultValue` and the typing for `argument()`. Arguably adding
`string[]` to the type, so it is `string[] | string | boolean` would be
more useful, because without a coerce function the default value must
be a string, boolean, or array of strings.

typings/index.d.ts

@@ -523,21 +523,21 @@ export class Command {
    *
    * @returns `this` command for chaining
    */
-  option(flags: string, description?: string, defaultValue?: string | boolean): this;
+  option(flags: string, description?: string, defaultValue?: unknown): this;
   option<T>(flags: string, description: string, fn: (value: string, previous: T) => T, defaultValue?: T): this;
   /** @deprecated since v7, instead use choices or a custom function */
-  option(flags: string, description: string, regexp: RegExp, defaultValue?: string | boolean): this;
+  option(flags: string, description: string, regexp: RegExp, defaultValue?: unknown): this;
 
   /**
    * Define a required option, which must have a value after parsing. This usually means
    * the option must be specified on the command line. (Otherwise the same as .option().)
    *
    * The `flags` string contains the short and/or long flags, separated by comma, a pipe or space.
    */
-  requiredOption(flags: string, description?: string, defaultValue?: string | boolean): this;
+  requiredOption(flags: string, description?: string, defaultValue?: unknown): this;
   requiredOption<T>(flags: string, description: string, fn: (value: string, previous: T) => T, defaultValue?: T): this;
   /** @deprecated since v7, instead use choices or a custom function */
-  requiredOption(flags: string, description: string, regexp: RegExp, defaultValue?: string | boolean): this;
+  requiredOption(flags: string, description: string, regexp: RegExp, defaultValue?: unknown): this;
 
   /**
    * Factory routine to create a new unattached option.