feat: support env var as default flag value

[molisani]

Resolves #62

Describe your changes
Expose functionality for application developers to pull the default value for a flag from an environment variable. This was already possible manually, but integrating it as a first-class feature allows for better formatting of error messages and the ability to "redact" the value in Stricli-generated output (for secrets).

Testing performed
Added several new test cases and updated the formatting baselines to achieve complete coverage.

Additional context
One side-effect is that CommandContext["process"] is now StricliProcess rather than just WritableStreams to allow env to be defined. The differences between the two types are just optional properties so there shouldn't be any issues.

[mkubilayk]

Can we also add a redact: true test?

[molisani]

Ah good catch, didn't have a case for redact in the formatting baselines.

[molisani]

Oh at the moment there isn't any difference in the help text when redact: true is enabled. Should there be?

    [--requiredBoolean/--noRequiredBoolean]  required boolean flag           [default = <from env REQUIRED_BOOLEAN>]
 -h --help                                   Print help information and exit

[molisani]

At the moment the help text doesn't display the value of the environment variable, only the name. While I think it would be incredibly handy to print the value (or redacted value) we also still need to print the name and all of that takes up a lot of space. Not sure what the best display would be yet.

[molisani]

Which of these options would be the best for displaying the default value? Right now we have the last one, which has the added benefit of not needing to read from the process to build the help text.

    [--flag]  some flag           [default = ACTUAL_VALUE]
    [--flag]  some flag           [default = ACTUAL_VALUE <from env REQUIRED_BOOLEAN>]
    [--flag]  some flag           [default = <from env REQUIRED_BOOLEAN>]

With the following for redact: true

    [--flag]  some flag           [default = ************]
    [--flag]  some flag           [default = ************ <from env REQUIRED_BOOLEAN>]
    [--flag]  some flag           [default = <from env REQUIRED_BOOLEAN>]

[mkubilayk]

My preference would be option (2) followed by (1) if length is a concern.

[molisani]

Sounds good, we can do 2 and still have the backout to 3 if there is no value set.

[mkubilayk]

My preference on formatting: #75 (comment).

[molisani]

The output now looks like this depending on value/config:

  [--flag]  flag brief    [default = env:ENV_VAR_NAME]
  [--flag]  flag brief    [default = "" | env:ENV_VAR_NAME]
  [--flag]  flag brief    [default = SOME_VALUE | env:ENV_VAR_NAME]
  [--flag]  flag brief    [default = ██████████ | env:ENV_VAR_NAME]

I switched the redact character from * to █ so that it was even more obvious that the value is purposefully hidden.

[molisani]

One problem with extending "default" to include environment variable values is that with normal defaults there is always a value but in the case where the variable is undefined then there is no value. Does it make sense to keep it as an extension of "default" or to split it off so that we can read from the env var and also have a default value?