Add `unbind-default-keys` config option

[Lindenk]

Closes #2720.

There were a few changes I made to reduce confusion with some function/method locations and naming:

• helix_term::keymap::merge_keys was moved to helix_term::config::Config::merge_keys as a method because it only operates on and changes Config, and was modified to take a set of bindings to merge with itself. New functions were also added to handle the new optional behavior.
• Config::load now is the single source of truth for loading and merging configs, which was previously also handled in helix_term::application::new in a slightly different but mostly redundant manner.
• Default is removed from Config and replaced with Config::new and Config::with_default_keys. new returns a Config with no bindings while with_default_keys returns a Config with the default bindings (which is the same behavior as the old default). This to prevent confusion between a default rust struct and helix's default keybindings as these terms were previously conflated.
• Config::keys no longer loads default bindings through serde when no keys are found as this was always redundant.
• helix_term::keymap::Keymaps::get no longer crashes when a mode isn't found. This will also help with future mode additions (likely through plugins down the line).

Let me know if any of these changes should be handled differently

[the-mikedavis]

Could you add some notes on using this to the docs? https://github.com/helix-editor/helix/tree/f37ffaa3a1c7754238029390db00148ec8105874/book/src

[Lindenk]

No problem. If you'd like me to include a new subsection or larger blurb, or if there's a better place to provide this specific documentation, let me know

[patrick-kidger]

Is there any movement on this? I'd really like to try out this feature.

[Lindenk]

I updated the feature to the current version of helix and addressed the above changes. Now it only adds the config option unbind_default_keys

[the-mikedavis]

https://github.com/helix-editor/helix/pull/9384#issuecomment-2065534491, https://github.com/helix-editor/helix/issues/2720#issuecomment-2065533370 - we want to solve this when we switch config away from TOML rather than adding a new config option now