Feature request: Multi-file selection for batch staging/unstaging

[kgfly]

Problem

Currently, toggle_stage_entry only works on a single file. There's no way to select multiple files and then stage/unstage them as a batch.

Current behavior
Navigate to file 1, press -: file 1 staged
Navigate to file 2, press -: file 2 staged
Each file must be staged individually. There's no visual selection mode.

Desired behavior
Similar to how telescope-file-browser.nvim and other file managers work:
Navigate to file 1, press Tab key (or similar): file 1 selected (visually marked)
Navigate to file 2, press Tab Key file 2 selected
Press - or s : both selected files are staged

Proposed implementation
Add a toggle_selection action that marks files without staging
Add a stage_selected action that stages all marked files
Visual indicator (highlight/icon) for selected files
Optional: to select all, to clear selection

Thanks.

[kgfly]

I have raised a PR #600. Please review