Add default_split tunable to control <C-W>f split direction

[stevenpollack]

Summary

Closes #415.

<C-W>f (open item under cursor in a split) currently hardcodes a horizontal split. Users who prefer vertical splits had to work around this with a macro like "f" = "<C-w>f<C-w><S-l>".

This PR adds a default_split tunable ("horizontal" by default, preserving current behaviour) that, when set to "vertical", overrides <C-W>f and <C-W><C-F> to open in a vertical split instead.

Config usage:

[settings]
default_split = "vertical"

Implementation notes

• SplitDirection enum added to config.rs, following the same #[serde(rename_all = "lowercase")] pattern as UserDisplayStyle.
• ApplicationSettings::setup() in keybindings.rs calls bindings.add_mapping for the two affected key sequences when default_split = "vertical". This works because the underlying keybindings crate's upsert_node overwrites a node's action when a later registration targets an already-registered key sequence — so it cleanly overrides the horizontal binding that modalkit registers during VimBindings::setup.
• No modalkit changes required.

Test plan

• Build with cargo build
• Run cargo test --locked
• With default_split = "vertical" in config, confirm <C-W>f on a room in the room list opens it in a vertical split
• Without the setting (or with "horizontal"), confirm existing behaviour is unchanged

[VAWVAW]

Please add documentation to docs/iamb.5.

[stevenpollack]

@VAWVAW done! Sorry for the omission. Completely slipped my eye.

[stevenpollack]

bump for freshness.

[VAWVAW]

This is a really slow moving project. It might be a few weeks or months until you get a reaction from a maintainer.

[stevenpollack]

This is a really slow moving project. It might be a few weeks or months until you get a reaction from a maintainer.

fair enough! thanks for the head's up. =)