Add command to list possible hotkeys for current mode

Given the following skhdrc:

```sh
a : echo "do this"

:: mymode
ctrl - m ; mymode
mymode < a : echo "in mymode – do this"
mymode < b : echo "in mymode – do that"
```

I'd like to be able to perform something like the following commands:

```sh
# in default mode
# `--show-options` is a new command which prints the possible hotkeys for the current state of skhd.
$ skhd --show-options
a : echo "do this"

# user presses `ctrl - m` and enters `mymode`
$ skhd --show-options
mymode < a : echo "in mymode – do this"
mymode < b : echo "in mymode – do that"
```

Let me know if the behavior is not clear and I can try to explain more.

I'm looking for this kind of command to help me write a GUI helper that shows me what commands I can invoke. I could write this as a separate application, but it seems like I would need to maintain a separate parser, and I would need to be able to track skhd's active mode to return only the relevant results.

I think my ideal output for this command would be JSON corresponding to the [`hotkey` struct](https://github.com/koekeishiya/skhd/blob/311328d8bfd001c57e135b1fda6267ea1b68df99/src/hotkey.h#L71) to avoid needing any further parsing.

Is this something you would want in skhd? I don't have a lot of C experience, so I thought I would ask before taking time to dive in.

Start with the requested --show-options command and inspect the hotkey struct in src/hotkey.h. Trace how skhd represents the current mode and parses the skhdrc example, then determine how output should expose only active hotkeys, ideally as JSON. Done means the command reports the relevant hotkeys in default and mymode states without requiring a separate parser.

Written by the indexing model from the issue text.