Add support for completing `--flag value<TAB>`, `-fbar<TAB>`, `-f=bar<TAB>` and `-f bar<TAB>`

[shannmu]

Related issues:

• https://github.com/clap-rs/clap/issues/3920
• https://github.com/clap-rs/clap/issues/3166

In this PR, I modified some logic in the complete function:

• Added CompletionState to record the current parsing state during completion parsing. When completion is triggered with target_cursor equal to cursor, the completion function will be chosen based on the CompletionState.
• Modified the priority handling in the complete function for arg over subcommand, optional long flag, escape, negative number, optional short flag, stdio, and empty.
• Modified the priority handling in complete_arg for arg, with the same priority as above.

There are some TODOs in this PR that I will implement later, including:

• Long flag, short flag, and alias completion for subcommand
• Support for args_conflicts_with_subcommands
• Support for subcommand_precedence_over_arg
• Support for multicall

[shannmu]

The current code hasn't been tested a lot. I've just done some quick tests using the dynamic program from the tests. Also, for the CI, I'll rebase onto clap's master branch later and modify the commit message and a few other things to meet the CI requirements.

[shannmu]

I need to split this PR into smaller PRs for easier discussion and review. How should I handle the situation where one resulting PR might be based on another PR?

[epage]

I need to split this PR into smaller PRs for easier discussion and review. How should I handle the situation where one resulting PR might be based on another PR?

Either hold off on the follow up PRs or put them up, note the dependency, and rebase once the other PR is merged.