Typings could be tighter for default value parameter for .option()

[shadowspawn]

Consider:

.option(flags, description, defaultValue)

Commander types this to match the possible option value types (tj/commander.js#1721 (comment)), leaving aside coercion:

defaultValue?: string | boolean | string[]

which allow options defined with matching defaults like:

.option('--string <value>', 'description, 'DEFAULT')  // string
.option('--boolean', 'description', false) // boolean
.option('--variadic <value...>, ['DEFAULT']) // string[]

In extra-typings we could restrict the default type to match the flag type?

[shadowspawn]

A side-note: my realisation reviewing the Pull Request is that the issue is not really that we were allowing mixed types, it is more that some of the mixed types silently produced bogus typings for the parsed options (.opts()), because the parsing fell through gaps in the expected combinations.