Add `editor::BlameHover` action for triggering the blame popover via keyboard

[Daste745]

Make the git blame popover available via the keymap by making it an action. The blame popover stays open after being shown via the action, similar to the editor::Hover action.

I added a default vim-mode key binding for g b, which goes in hand with g h for hover. I'm not sure what the keybind would be for regular layouts, if any would be set by default.

I'm opening this as a draft because I coludn't figure out a way to position the popover correctly above/under the cursor head. I saw some uses of content_origin in other places for calculating absolute pixel positions, but I'm not sure how to make use of it here without doing a big refactor of the blame popover code 🤔. I would appreciate some help/tips with positioning, because it seems like the last thing to implement here.

Opening as a draft for now because I think without the correct positioning this feature is not complete.

Closes https://github.com/zed-industries/zed/discussions/26447

Release Notes:

• Added editor::BlameHover action for showing the git blame popover under the cursor. By default bound to g h in vim mode.