Add toggleable Git blame view for the displayed file

[kyoto7250]

Description:

Currently, the file content is displayed, but it would be useful to show Git blame information on demand.

What you need to do:

1. Add a key command (e.g., Shift + B) to toggle showing the blame view, displaying the committer and commit ID for each line.
2. Pressing the same key command again should close the blame view and return to the normal file view.

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