feat: add support for custom help triggers

[aminya]

Fixes #30
Related to #53
Related to #119

This adds support for custom help triggers using #[argh(help_triggers("-h", "--help", "help"))].

For example,

/// Height options
#[derive(FromArgs)]
#[argh(help_triggers("-h", "--help", "help"))]
struct Height {
    /// how high to go
    #[argh(option)]
    height: usize,
}

Allows triggering help with -h, --help or help

Usage: test_arg_0 --height <height>

Height options

Options:
  --height          how high to go
  -h, --help, help  display usage information

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[aminya]

@googlebot I signed it!

[tristan-f-r]

LGTM!

[sadmac7000]

I don't think we can change something this fundamental without having at least the option for the old behavior. My preference would be that it default to off.

[aminya]

@sadmac7000 I submitted this pull request in 2021. After these years, is it worth making this simple change configurable? If I was the maintainer, I would have merged it and bumped up the major version.

[sadmac7000]

argh exists specifically to implement the Fuchsia command line argument guidelines. Breaking behaviors outside of those guidelines need more than just a major revision bump. They need an update to the guidelines or to be disabled by default.

[aminya]

@sadmac7000 Where is the Fuschia documentation so that I can make the same pull request for that?

[sadmac7000]

The guidelines are documented in Google's git hosting here: https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/docs/development/api/cli.md

[aminya]

@sadmac7000 Reimplemented the help trigger logic to make it configurable through attributes.

[sadmac7000]

Looks good. Looks like there's a merge conflict. Mind rebasing?

[aminya]

@sadmac7000 You're welcome. The branch is already rebased. I noticed the GitHub UI can be outdated, so maybe refresh your page.

[sadmac7000]

It's still claiming the branch cannot be rebased due to conflicts and won't let me merge. I refreshed a few times. I'll look again later in case it's a transient error.

[aminya]

@sadmac7000 I gave it a shock. Check if it resolved the issue. Haha

[sadmac7000]

I had to pull it down and manually rearrange it. In the future, please rebase your changes when you need to update rather than merging in upstream. It confuses github.