Add support for customizable key bindings using `key_bind.ron`

[kyoto7250]

Description:

Since gview is a local TUI application, it would be great to allow users to customize the default key bindings.

What you need to do:

1. Add support for the ron format to read key bindings from a configuration file.
2. Load the key bindings from a file like .config/gview/key_bind.ron if it exists.

• If the file does not exist, use the default key bindings.
• If loading fails, print an error message and exit.

3. Refactor the current key bindings into a pub struct KeyBind (or similar) so that components can read from it and behave accordingly.

This is a somewhat complex change affecting the whole codebase, but I have implemented the same feature in a previous TUI project. Feel free to refer to that implementation as a guide:

ref: kyoto7250/zhobo#20

If you have any questions, please comment on this issue!