feat(parser): accept boolean literal with env vars

[rami3l]

Closes #2539.

This PR allows boolean literals following the Python strtobool convention (case-insensitive):

True values are y, yes, t, true, on and 1; false values are n, no, f, false, off and 0.

• A false literal or an absent env var will cause the flag to be considered absent.
• If the env var gets a true literal, then the flag is considered present by the parser.
• If the boolean literal is anything else, the parser returns an error.
• If the env var gets anything else, then the flag is considered present by the parser (suggested by @pksunkara).

Questions

• Should we adjust the APIs so that other conventions are also allowed?
  • The pseudo-flag pattern should be used in this case.
• How will this adjustment interact with the rest of the APIs?

[pksunkara]

This needs to be behind a new arg setting

[rami3l]

@pksunkara Do you mean enabling the new behavior with a flag?

I understand that making the new behavior as the default will be a breaking change.

However, the main issue that I personally want to address is that, ENV_VAR=false is currently considered as true. It seems to be a highly possible misuse of the current API.

By keeping the old behavior by default, the misuse will probably continue. What do you think could be the appropriate way of handling this?

[pksunkara]

You are right. But I think any other random value should still be reduced to true instead of raising error. What do you think?

[rami3l]

@pksunkara Thanks for your advice!

I assume that one can still use something like the pseudo-flag pattern if they do want an error. In that case, what you said could be a better default.

Sorry, I'm a bit occupied right now. I will catch up with the refactoring a bit later :)

[pksunkara]

Yup. Sounds good. Will wait for an update then.

[rami3l]

@pksunkara Rebased. Also replaced nested braces with early returns for readability.

Suggestion: Since the pseudo-flag pattern seems to play such an important role in both env flags and CLI flags, maybe we should try to improve its visibility.