Refactor color scheme handling and key bindings initialization

[AnthonySmithDev]

Description:
This PR introduces several improvements to the plugin's initialization logic:

1. Color Scheme Fixes

   • Previously, if users only set config.color_scheme without config.colors, the plugin would ignore the scheme and fall back to default colors.
   • Now, if config.color_scheme is defined, it properly loads the built-in scheme instead of defaulting.
   • This ensures users who rely on color_scheme (a common Wezterm setting) get the correct colors without needing to manually define config.colors.

2. Optional Default Keybinds

   • Before, the plugin automatically added default keybindings (like ALT+n, ALT+u, ALT+c) even if users didn’t want them.
   • Now, keybind initialization is moved into a separate set_default_keys(config) function, which must be called explicitly.
   • This gives users full control—they can:
     • Skip calling it to avoid default keybinds entirely.
     • Call it once to apply defaults.
     • Modify the keys afterward if they want partial overrides.

Why This Matters

• Fixes silent color issues – Users who only set color_scheme (very common) no longer get unexpected default colors.
• More flexible keybinds – Users can now opt-in to defaults instead of having them forced.

Migration Notes

• No changes needed if you already define config.colors manually.
• If you want the old default keybinds, add set_default_keys(config) after applying the plugin.
• If you don’t want default keybinds, do nothing—they won’t be added automatically anymore.

[MLFlexer]

Thank you very much for your contribution!
I think these are some great changes, the only thing I'm missing for this PR is to add the set_default_keys function to the documentation in the README, maybe in the preset section.

Once that is in, then It can be merged 😄