Add option filter to +show-config action

[korikhin]

What do you want to change?

I'd like to implement feature #2486 which allows more granular config printing for the +show-config action.

Please see the comparison. This implementation adds the ability to provide a list of option names for output filtering. Pattern matching is not implemented. Options preserve their original behaviour.

# No filters. Prints the full user config.
ghostty +show-config
ghostty +show-config --

# No match. Prints nothing (exit code 0).
ghostty +show-config -- non-existent-option

# Prints identical output. Entries deduplicated and order-independent.
# Non-existent names ignored, and so are empty or whitespace-only arguments.
ghostty +show-config -- font-family font-size
ghostty +show-config -- font-size non-existent-option font-family font-size '' ' '

# Prints default values.
ghostty +show-config --default -- font-family font-size

In order to understand how argument parsing and +show-config action work I examined the following related source code:

• src/config (mostly Config.zig and key.zig);
• src/cli (mostly args.zig and ghostty.zig). I also looked into +ssh, +new-tab, and +new-window action source code to take it as a reference as they all use parseManuallyHook.

I'm new to Zig so I also decided to take this feature as a learning opportunity. I hope I didn't miss anything.

AI Disclosure: I asked Claude (Sonnet 5, free version) whether it is a good idea to introduce a -- delimiter, because it strays from the original feature request, but simplifies parsing (CLI design decision). I also asked Claude to help me phrase the documentation comments. The code is hand-rolled.

Why do you want to make this change?

The motivation for this change is explained in the original feature request (#2486). This is also useful for me personally. I usually reach for the official website to find documentation for a particular setting, which is not super convenient sometimes.

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.

[korikhin]

Hi @jparise! Would you be interested in taking a look, if you have the time?

[jcollie]

!vouch