feat(clap_complete): Support -fbar and -f=bar in native completions

[shannmu]

related issue: #3920

This is a later PR based on #5539

What is left

Error parsing for --flag=bar in bash
more details:

clap/clap_complete/src/dynamic/shells/mod.rs

Lines 36 to 48 in 43e7368

	pub struct CompleteArgs {
	/// Specify shell to complete for
	#[arg(long)]
	shell: Shell,
	/// Path to write completion-registration to
	#[arg(long, required = true)]
	register: Option<std::path::PathBuf>,
	#[arg(raw = true, hide_short_help = true, group = "complete")]
	comp_words: Vec<OsString>,
	}

The comp_words will be ['command', '--flag', '=', 'value'] rather than ['command', '--flag=value'] because of COMP_WORDBREAKS.

[epage]

Marked as draft until #5539 is merged just to be clear that this is blocked on it

[epage]

Error parsing for --flag=bar in bash

As the draft status was removed, I'm assuming this isn't a blocker for merge? Could you give some detail on why?

[shannmu]

As the draft status was removed, I'm assuming this isn't a blocker for merge? Could you give some detail on why?

This error is unrelated to the Rust part of the code because it is caused by COMP WORDBREAKS. COMP WORDBREAKS affects the behavior of bash completion and is a shell-specific thing, whereas the purpose of this PR is to implement shell-agnostic completion feature.
"What is left" is intended to remind us that #3920 still has some unresolved issues, and perhaps we should address it as a separate issue.

Detail for COMP_WORDBREAKS

COMP_WORDBREAKS
The set of characters that the Readline library treats as word separators when performing word completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset.