Add editor::BlameHover action for triggering the blame popover via keyboard (#32096)

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 `ctrl-k ctrl-b` and to `g h` in
vim mode.

assets/keymaps/default-linux.json

@@ -483,6 +483,7 @@
       "ctrl-k ctrl-d": ["editor::SelectNext", { "replace_newest": true }], // editor.action.moveSelectionToNextFindMatch  / find_under_expand_skip
       "ctrl-k ctrl-shift-d": ["editor::SelectPrevious", { "replace_newest": true }], // editor.action.moveSelectionToPreviousFindMatch
       "ctrl-k ctrl-i": "editor::Hover",
+      "ctrl-k ctrl-b": "editor::BlameHover",
       "ctrl-/": ["editor::ToggleComments", { "advance_downwards": false }],
       "f8": ["editor::GoToDiagnostic", { "severity": { "min": "hint", "max": "error" } }],
       "shift-f8": ["editor::GoToPreviousDiagnostic", { "severity": { "min": "hint", "max": "error" } }],