ArgParser rewrite

[YoshiRulz]

I'd still like to improve the documentation further, but this is functional already. Documentation improved. If you have scripts for e.g. encoding please test them.

Don't merge without checking the commit hash in README.md (it should be of the commit immediately before the readme change).

[Morilli]

If we do this, is there a way to get rid of the ParsedCLIFlags struct as well? Feels like unnecessary duplication.

[YoshiRulz]

If we do this, is there a way to get rid of the ParsedCLIFlags struct as well? Feels like unnecessary duplication.

Yes, by exposing the Options, returning the ParseResult, and moving the GetValueForOption calls throughout MainForm. I will try to do that for this PR. edit: Actually, MainForm only uses a few flags on init, most of them are read in (methods called from) the main loop, so they'd need to be memoised anyway. And errors from our manual parsing would possibly happen later. So I don't think it's a good idea.

[YoshiRulz]

Should --help group the flags semantically? (Re-ordering the list is trivial, unfortunately adding headers isn't.) I chose to list them alphabetically since that's what I'm used to from Unix tools. Thinking about this again, I'm always annoyed when I run appname --help and it's not in alphabetical order (youtube-dl), so I'm now decidedly against that.

[Morilli]

This is probably an improvement for end-users (well, assuming you get --help to actually display stuff on windows) and about neutral for understanding code flow (it's a complete mess still).

[Morilli]

This doesn't actually work on windows release builds due to the WinExe target, so this will need some consideration.

[YoshiRulz]

I've fixed this by caveating it with "on Linux".

(The CommandLineBuilder.UseHelp(Action<HelpContext>) overload does seem to run on-demand i.e. only when --help is passed, if you wanted to look into that Win32 Console hack post-merge.)

[YoshiRulz]

Rebased, only differences are:

• readme edits now reflect --help being Linux-only, and that the next release will be 2.10; and
• NuGet dep was made non-propagating. Apparently this stopped the assembly from being copied to the output. Fixed with 09f0375.

There are no actionable review comments left (edit: for me to do, that is; the possibility for --help on Windows remains), so I'll leave about a day for final comments before merging this.
(I'd like this in 2.10-RC1 to give the greatest chance of catching regressions. Yesterday on Discord we discussed getting to an RC soon-ish, which I believe we can do!)