Deprecate support for is_flag and flag_value parameters

[svlandeg]

Privileged issue

• I'm @tiangolo or he asked me directly to create an issue here.

Issue Content

Typer's code base currently contains is_flag and flag_value parameters like Click does. However, the current behaviour is inconsistent with Click's behaviour, and not documented.

In discussion with @tiangolo, we've come to the conclusion that these parameters are pretty confusing overall to work with. There are essentially two defaults (one set in code, and one set by flag_value) and then an option to set an arbitrary value to something that is a "flag". We feel that any behaviour obtained by this combination of parameters can probably be achieved easily with some internal logic and more straightforward (and properly typed) options/arguments.

As such, we've decided to remove these options from the code base as they're not really functional right now anyway.

[svlandeg]

Related discussions:

• How create an option with an optional argument (e.g. a flag with an optional value) #608
• Option optional value doesn't work because `is_flag` is not passed to click properly #873
• 🐛 Ensure that the value of is_flag is passed correctly to Click #872